diff --git a/src/app.rs b/src/app.rs
index 1023b97..ffabcb4 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -10,7 +10,7 @@ use crate::commands::AppCommand;
 use crate::engine::{
     LinkState, PatternChange, PatternSnapshot, SeqCommand, SequencerSnapshot, StepSnapshot,
 };
-use crate::model::{self, Bank, Pattern, Rng, ScriptEngine, StepContext, Variables};
+use crate::model::{self, Bank, Dictionary, Pattern, Rng, ScriptEngine, StepContext, Variables};
 use crate::page::Page;
 use crate::services::pattern_editor;
 use crate::settings::Settings;
@@ -35,6 +35,7 @@ pub struct App {
     pub metrics: Metrics,
     pub script_engine: ScriptEngine,
     pub variables: Variables,
+    pub dict: Dictionary,
     pub rng: Rng,
     pub live_keys: Arc<LiveKeyState>,
     pub clipboard: Option<arboard::Clipboard>,
@@ -47,8 +48,9 @@ pub struct App {
 impl App {
     pub fn new() -> Self {
         let variables = Arc::new(Mutex::new(HashMap::new()));
+        let dict = Arc::new(Mutex::new(HashMap::new()));
         let rng = Arc::new(Mutex::new(StdRng::seed_from_u64(0)));
-        let script_engine = ScriptEngine::new(Arc::clone(&variables), Arc::clone(&rng));
+        let script_engine = ScriptEngine::new(Arc::clone(&variables), Arc::clone(&dict), Arc::clone(&rng));
         let live_keys = Arc::new(LiveKeyState::new());
 
         Self {
@@ -63,6 +65,7 @@ impl App {
 
             metrics: Metrics::default(),
             variables,
+            dict,
             rng,
             live_keys,
             script_engine,
diff --git a/src/engine/sequencer.rs b/src/engine/sequencer.rs
index 58e8149..2775e6f 100644
--- a/src/engine/sequencer.rs
+++ b/src/engine/sequencer.rs
@@ -6,7 +6,7 @@ use std::time::Duration;
 
 use super::LinkState;
 use crate::config::{MAX_BANKS, MAX_PATTERNS};
-use crate::model::{ExecutionTrace, Rng, ScriptEngine, StepContext, Variables};
+use crate::model::{Dictionary, ExecutionTrace, Rng, ScriptEngine, StepContext, Variables};
 use crate::state::LiveKeyState;
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
@@ -177,6 +177,7 @@ pub fn spawn_sequencer(
     link: Arc<LinkState>,
     playing: Arc<std::sync::atomic::AtomicBool>,
     variables: Variables,
+    dict: Dictionary,
     rng: Rng,
     quantum: f64,
     live_keys: Arc<LiveKeyState>,
@@ -197,6 +198,7 @@ pub fn spawn_sequencer(
                 link,
                 playing,
                 variables,
+                dict,
                 rng,
                 quantum,
                 shared_state_clone,
@@ -291,6 +293,7 @@ fn sequencer_loop(
     link: Arc<LinkState>,
     playing: Arc<std::sync::atomic::AtomicBool>,
     variables: Variables,
+    dict: Dictionary,
     rng: Rng,
     quantum: f64,
     shared_state: Arc<Mutex<SharedSequencerState>>,
@@ -298,7 +301,7 @@ fn sequencer_loop(
 ) {
     use std::sync::atomic::Ordering;
 
-    let script_engine = ScriptEngine::new(Arc::clone(&variables), rng);
+    let script_engine = ScriptEngine::new(Arc::clone(&variables), dict, rng);
     let mut audio_state = AudioState::new();
     let mut pattern_cache = PatternCache::new();
     let mut runs_counter = RunsCounter::new();
diff --git a/src/main.rs b/src/main.rs
index 7fad8b4..d4605ee 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -102,6 +102,7 @@ fn main() -> io::Result<()> {
         Arc::clone(&link),
         Arc::clone(&playing),
         Arc::clone(&app.variables),
+        Arc::clone(&app.dict),
         Arc::clone(&app.rng),
         settings.link.quantum,
         Arc::clone(&app.live_keys),
diff --git a/src/model/forth.rs b/src/model/forth.rs
index c5e5f0c..7e04289 100644
--- a/src/model/forth.rs
+++ b/src/model/forth.rs
@@ -35,6 +35,7 @@ impl StepContext {
 }
 
 pub type Variables = Arc<Mutex<HashMap<String, Value>>>;
+pub type Dictionary = Arc<Mutex<HashMap<String, Vec<Op>>>>;
 pub type Rng = Arc<Mutex<StdRng>>;
 
 #[derive(Clone, Debug)]
@@ -212,6 +213,7 @@ pub enum Op {
     LocalCycleEnd,
     Echo,
     Necho,
+    Apply,
 }
 
 pub enum WordCompile {
@@ -1606,6 +1608,29 @@ pub const WORDS: &[Word] = &[
         example: "1 reset",
         compile: Param,
     },
+    // Quotation execution
+    Word {
+        name: "apply",
+        stack: "(quot --)",
+        desc: "Execute quotation unconditionally",
+        example: "{ 2 * } apply",
+        compile: Simple,
+    },
+    // Word definitions
+    Word {
+        name: ":",
+        stack: "( -- )",
+        desc: "Begin word definition",
+        example: ": kick \"kick\" s emit ;",
+        compile: Simple,
+    },
+    Word {
+        name: ";",
+        stack: "( -- )",
+        desc: "End word definition",
+        example: ": kick \"kick\" s emit ;",
+        compile: Simple,
+    },
 ];
 
 fn simple_op(name: &str) -> Option<Op> {
@@ -1672,6 +1697,7 @@ fn simple_op(name: &str) -> Option<Op> {
         "for" => Op::For,
         "echo" => Op::Echo,
         "necho" => Op::Necho,
+        "apply" => Op::Apply,
         _ => return None,
     })
 }
@@ -1751,7 +1777,7 @@ fn parse_interval(name: &str) -> Option<i64> {
     Some(simple)
 }
 
-fn compile_word(name: &str, span: Option<SourceSpan>, ops: &mut Vec<Op>) -> bool {
+fn compile_word(name: &str, span: Option<SourceSpan>, ops: &mut Vec<Op>, dict: &Dictionary) -> bool {
     for word in WORDS {
         if word.name == name {
             match &word.compile {
@@ -1762,7 +1788,7 @@ fn compile_word(name: &str, span: Option<SourceSpan>, ops: &mut Vec<Op>) -> bool
                 }
                 Context(ctx) => ops.push(Op::GetContext((*ctx).into())),
                 Param => ops.push(Op::SetParam(name.into())),
-                Alias(target) => return compile_word(target, span, ops),
+                Alias(target) => return compile_word(target, span, ops, dict),
                 Probability(p) => {
                     ops.push(Op::PushFloat(*p, None));
                     ops.push(Op::ChanceExec);
@@ -1810,6 +1836,12 @@ fn compile_word(name: &str, span: Option<SourceSpan>, ops: &mut Vec<Op>) -> bool
         return true;
     }
 
+    // User-defined words from dictionary
+    if let Some(body) = dict.lock().unwrap().get(name) {
+        ops.extend(body.iter().cloned());
+        return true;
+    }
+
     false
 }
 
@@ -1905,7 +1937,7 @@ fn tokenize(input: &str) -> Vec<Token> {
     tokens
 }
 
-fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
+fn compile(tokens: &[Token], dict: &Dictionary) -> Result<Vec<Op>, String> {
     let mut ops = Vec::new();
     let mut i = 0;
     let mut pipe_parity = false;
@@ -1916,7 +1948,7 @@ fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
             Token::Float(f, span) => ops.push(Op::PushFloat(*f, Some(*span))),
             Token::Str(s, span) => ops.push(Op::PushStr(s.clone(), Some(*span))),
             Token::QuoteStart(start_pos) => {
-                let (quote_ops, consumed, end_pos) = compile_quotation(&tokens[i + 1..])?;
+                let (quote_ops, consumed, end_pos) = compile_quotation(&tokens[i + 1..], dict)?;
                 i += consumed;
                 let body_span = SourceSpan { start: *start_pos, end: end_pos + 1 };
                 ops.push(Op::Quotation(quote_ops, Some(body_span)));
@@ -1926,7 +1958,13 @@ fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
             }
             Token::Word(w, span) => {
                 let word = w.as_str();
-                if word == "|" {
+                if word == ":" {
+                    let (consumed, name, body) = compile_colon_def(&tokens[i + 1..], dict)?;
+                    i += consumed;
+                    dict.lock().unwrap().insert(name, body);
+                } else if word == ";" {
+                    return Err("unexpected ;".into());
+                } else if word == "|" {
                     if pipe_parity {
                         ops.push(Op::LocalCycleEnd);
                     } else {
@@ -1934,7 +1972,7 @@ fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
                     }
                     pipe_parity = !pipe_parity;
                 } else if word == "if" {
-                    let (then_ops, else_ops, consumed, then_span, else_span) = compile_if(&tokens[i + 1..])?;
+                    let (then_ops, else_ops, consumed, then_span, else_span) = compile_if(&tokens[i + 1..], dict)?;
                     i += consumed;
                     if else_ops.is_empty() {
                         ops.push(Op::BranchIfZero(then_ops.len(), then_span, None));
@@ -1945,7 +1983,7 @@ fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
                         ops.push(Op::Branch(else_ops.len()));
                         ops.extend(else_ops);
                     }
-                } else if !compile_word(word, Some(*span), &mut ops) {
+                } else if !compile_word(word, Some(*span), &mut ops, dict) {
                     return Err(format!("unknown word: {word}"));
                 }
             }
@@ -1956,7 +1994,7 @@ fn compile(tokens: &[Token]) -> Result<Vec<Op>, String> {
     Ok(ops)
 }
 
-fn compile_quotation(tokens: &[Token]) -> Result<(Vec<Op>, usize, usize), String> {
+fn compile_quotation(tokens: &[Token], dict: &Dictionary) -> Result<(Vec<Op>, usize, usize), String> {
     let mut depth = 1;
     let mut end_idx = None;
 
@@ -1979,7 +2017,7 @@ fn compile_quotation(tokens: &[Token]) -> Result<(Vec<Op>, usize, usize), String
         Token::QuoteEnd(pos) => *pos,
         _ => unreachable!(),
     };
-    let quote_ops = compile(&tokens[..end_idx])?;
+    let quote_ops = compile(&tokens[..end_idx], dict)?;
     Ok((quote_ops, end_idx + 1, byte_pos))
 }
 
@@ -1991,6 +2029,30 @@ fn token_span(tok: &Token) -> Option<SourceSpan> {
     }
 }
 
+fn compile_colon_def(tokens: &[Token], dict: &Dictionary) -> Result<(usize, String, Vec<Op>), String> {
+    if tokens.is_empty() {
+        return Err("expected word name after ':'".into());
+    }
+    let name = match &tokens[0] {
+        Token::Word(w, _) => w.clone(),
+        _ => return Err("expected word name after ':'".into()),
+    };
+    let mut semi_pos = None;
+    for (i, tok) in tokens[1..].iter().enumerate() {
+        if let Token::Word(w, _) = tok {
+            if w == ";" {
+                semi_pos = Some(i + 1);
+                break;
+            }
+        }
+    }
+    let semi_pos = semi_pos.ok_or("missing ';' in word definition")?;
+    let body_tokens = &tokens[1..semi_pos];
+    let body_ops = compile(body_tokens, dict)?;
+    // consumed = name + body + semicolon
+    Ok((semi_pos + 1, name, body_ops))
+}
+
 fn tokens_span(tokens: &[Token]) -> Option<SourceSpan> {
     let first = tokens.first().and_then(token_span)?;
     let last = tokens.last().and_then(token_span)?;
@@ -1998,7 +2060,7 @@ fn tokens_span(tokens: &[Token]) -> Option<SourceSpan> {
 }
 
 #[allow(clippy::type_complexity)]
-fn compile_if(tokens: &[Token]) -> Result<(Vec<Op>, Vec<Op>, usize, Option<SourceSpan>, Option<SourceSpan>), String> {
+fn compile_if(tokens: &[Token], dict: &Dictionary) -> Result<(Vec<Op>, Vec<Op>, usize, Option<SourceSpan>, Option<SourceSpan>), String> {
     let mut depth = 1;
     let mut else_pos = None;
     let mut then_pos = None;
@@ -2027,13 +2089,13 @@ fn compile_if(tokens: &[Token]) -> Result<(Vec<Op>, Vec<Op>, usize, Option<Sourc
         let else_slice = &tokens[ep + 1..then_pos];
         let then_span = tokens_span(then_slice);
         let else_span = tokens_span(else_slice);
-        let then_ops = compile(then_slice)?;
-        let else_ops = compile(else_slice)?;
+        let then_ops = compile(then_slice, dict)?;
+        let else_ops = compile(else_slice, dict)?;
         (then_ops, else_ops, then_span, else_span)
     } else {
         let then_slice = &tokens[..then_pos];
         let then_span = tokens_span(then_slice);
-        let then_ops = compile(then_slice)?;
+        let then_ops = compile(then_slice, dict)?;
         (then_ops, Vec::new(), then_span, None)
     };
 
@@ -2045,14 +2107,16 @@ pub type Stack = Arc<Mutex<Vec<Value>>>;
 pub struct Forth {
     stack: Stack,
     vars: Variables,
+    dict: Dictionary,
     rng: Rng,
 }
 
 impl Forth {
-    pub fn new(vars: Variables, rng: Rng) -> Self {
+    pub fn new(vars: Variables, dict: Dictionary, rng: Rng) -> Self {
         Self {
             stack: Arc::new(Mutex::new(Vec::new())),
             vars,
+            dict,
             rng,
         }
     }
@@ -2091,7 +2155,7 @@ impl Forth {
         }
 
         let tokens = tokenize(script);
-        let ops = compile(&tokens)?;
+        let ops = compile(&tokens, &self.dict)?;
         self.execute(&ops, ctx, trace)
     }
 
@@ -2824,6 +2888,23 @@ impl Forth {
                     }
                     stack.push(selected);
                 }
+
+                Op::Apply => {
+                    let quot = stack.pop().ok_or("stack underflow")?;
+                    match quot {
+                        Value::Quotation(quot_ops, body_span) => {
+                            if let Some(span) = body_span {
+                                if let Some(trace) = trace_cell.borrow_mut().as_mut() {
+                                    trace.executed_spans.push(span);
+                                }
+                            }
+                            let mut trace_opt = trace_cell.borrow_mut().take();
+                            self.execute_ops(&quot_ops, ctx, stack, outputs, time_stack, cmd, trace_opt.as_deref_mut())?;
+                            *trace_cell.borrow_mut() = trace_opt;
+                        }
+                        _ => return Err("expected quotation".into()),
+                    }
+                }
             }
             pc += 1;
         }
diff --git a/src/model/mod.rs b/src/model/mod.rs
index c311afb..9ad274c 100644
--- a/src/model/mod.rs
+++ b/src/model/mod.rs
@@ -5,4 +5,4 @@ mod script;
 
 pub use file::{load, save};
 pub use project::{Bank, Pattern, PatternSpeed, Project};
-pub use script::{ExecutionTrace, Rng, ScriptEngine, SourceSpan, StepContext, Variables};
+pub use script::{Dictionary, ExecutionTrace, Rng, ScriptEngine, SourceSpan, StepContext, Variables};
diff --git a/src/model/script.rs b/src/model/script.rs
index d0f6bdd..bc6263b 100644
--- a/src/model/script.rs
+++ b/src/model/script.rs
@@ -1,15 +1,15 @@
 use super::forth::Forth;
 
-pub use super::forth::{ExecutionTrace, Rng, SourceSpan, StepContext, Variables};
+pub use super::forth::{Dictionary, ExecutionTrace, Rng, SourceSpan, StepContext, Variables};
 
 pub struct ScriptEngine {
     forth: Forth,
 }
 
 impl ScriptEngine {
-    pub fn new(vars: Variables, rng: Rng) -> Self {
+    pub fn new(vars: Variables, dict: Dictionary, rng: Rng) -> Self {
         Self {
-            forth: Forth::new(vars, rng),
+            forth: Forth::new(vars, dict, rng),
         }
     }
 
diff --git a/src/views/highlight.rs b/src/views/highlight.rs
index 97f5d29..b376cf0 100644
--- a/src/views/highlight.rs
+++ b/src/views/highlight.rs
@@ -55,7 +55,7 @@ const KEYWORDS: &[&str] = &[
     "zoom", "scale!", "stack", "echo", "necho", "for", "div", "each", "at", "pop", "adsr", "ad",
     "?", "!?", "<<", ">>", "|", "@", "!", "pcycle", "tempo!", "prob", "sometimes", "often",
     "rarely", "almostAlways", "almostNever", "always", "never", "coin", "fill", "iter", "every",
-    "gt", "lt",
+    "gt", "lt", ":", ";", "apply",
 ];
 const SOUND: &[&str] = &["sound", "s"];
 const CONTEXT: &[&str] = &[
diff --git a/tests/forth.rs b/tests/forth.rs
index e14f0f0..d0c95cb 100644
--- a/tests/forth.rs
+++ b/tests/forth.rs
@@ -42,3 +42,6 @@ mod notes;
 
 #[path = "forth/intervals.rs"]
 mod intervals;
+
+#[path = "forth/definitions.rs"]
+mod definitions;
diff --git a/tests/forth/definitions.rs b/tests/forth/definitions.rs
new file mode 100644
index 0000000..48435a7
--- /dev/null
+++ b/tests/forth/definitions.rs
@@ -0,0 +1,115 @@
+use super::harness::*;
+
+#[test]
+fn define_and_use_word() {
+    expect_int(": double 2 * ; 5 double", 10);
+}
+
+#[test]
+fn define_word_with_multiple_ops() {
+    expect_int(": triple dup dup + + ; 3 triple", 9);
+}
+
+#[test]
+fn define_word_using_another_user_word() {
+    expect_int(": double 2 * ; : quad double double ; 3 quad", 12);
+}
+
+#[test]
+fn redefine_word_overwrites() {
+    expect_int(": foo 10 ; : foo 20 ; foo", 20);
+}
+
+#[test]
+fn word_with_param() {
+    let outputs = expect_outputs(": loud 0.9 gain ; \"kick\" s loud emit", 1);
+    assert!(outputs[0].contains("gain/0.9"));
+}
+
+#[test]
+fn word_available_across_evaluations() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(": hi 42 ;", &ctx).unwrap();
+    f.evaluate("hi", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 42);
+}
+
+#[test]
+fn word_defined_in_one_forth_available_in_same() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(": ten 10 ;", &ctx).unwrap();
+    f.clear_stack();
+    f.evaluate("ten ten +", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 20);
+}
+
+#[test]
+fn unknown_word_errors() {
+    expect_error("nosuchword", "unknown word");
+}
+
+#[test]
+fn missing_semicolon_errors() {
+    expect_error(": foo 10", "missing ';'");
+}
+
+#[test]
+fn missing_name_errors() {
+    expect_error(":", "expected word name");
+}
+
+#[test]
+fn unexpected_semicolon_errors() {
+    expect_error(";", "unexpected ;");
+}
+
+#[test]
+fn apply_executes_quotation() {
+    expect_int("5 { 2 * } apply", 10);
+}
+
+#[test]
+fn apply_with_stack_ops() {
+    expect_int("3 4 { + } apply", 7);
+}
+
+#[test]
+fn apply_empty_stack_errors() {
+    expect_error("apply", "stack underflow");
+}
+
+#[test]
+fn apply_non_quotation_errors() {
+    expect_error("42 apply", "expected quotation");
+}
+
+#[test]
+fn apply_nested() {
+    expect_int("2 { { 3 * } apply } apply", 6);
+}
+
+#[test]
+fn define_word_containing_quotation() {
+    expect_int(": dbl { 2 * } apply ; 7 dbl", 14);
+}
+
+#[test]
+fn define_word_with_sound() {
+    let outputs = expect_outputs(": kick \"kick\" s emit ; kick", 1);
+    assert!(outputs[0].contains("sound/kick"));
+}
+
+#[test]
+fn define_word_with_conditional() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(": maybe-double dup 5 gt if 2 * then ;", &ctx).unwrap();
+    f.clear_stack();
+    f.evaluate("3 maybe-double", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 3);
+    f.clear_stack();
+    f.evaluate("10 maybe-double", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 20);
+}
diff --git a/tests/forth/harness.rs b/tests/forth/harness.rs
index 36e0619..204204a 100644
--- a/tests/forth/harness.rs
+++ b/tests/forth/harness.rs
@@ -1,6 +1,6 @@
 use rand::rngs::StdRng;
 use rand::SeedableRng;
-use cagire::model::forth::{Forth, Rng, StepContext, Value, Variables};
+use cagire::model::forth::{Dictionary, Forth, Rng, StepContext, Value, Variables};
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 
@@ -29,16 +29,20 @@ pub fn new_vars() -> Variables {
     Arc::new(Mutex::new(HashMap::new()))
 }
 
+pub fn new_dict() -> Dictionary {
+    Arc::new(Mutex::new(HashMap::new()))
+}
+
 pub fn seeded_rng(seed: u64) -> Rng {
     Arc::new(Mutex::new(StdRng::seed_from_u64(seed)))
 }
 
 pub fn forth() -> Forth {
-    Forth::new(new_vars(), seeded_rng(42))
+    Forth::new(new_vars(), new_dict(), seeded_rng(42))
 }
 
 pub fn forth_seeded(seed: u64) -> Forth {
-    Forth::new(new_vars(), seeded_rng(seed))
+    Forth::new(new_vars(), new_dict(), seeded_rng(seed))
 }
 
 pub fn run(script: &str) -> Forth {