diff --git a/doc/glossary.md b/doc/glossary.md
index 92b4689..48a022b 100644
--- a/doc/glossary.md
+++ b/doc/glossary.md
@@ -20,6 +20,10 @@
 applied to functions. See the [effects](/doc/note/effects.md) note for more
 details.
 
+Syntactically, effects are the exclamation and question marks, `!` and `?`, at
+function definitions and function calls. These denote impure and
+[coroutine](/doc/note/coroutines.md) functions.
+
 #### Fact
 
 A boolean expression (e.g. `x > y`) that happens to be true at a given point in