diff --git a/llvm/docs/Lexicon.rst b/llvm/docs/Lexicon.rst
--- a/llvm/docs/Lexicon.rst
+++ b/llvm/docs/Lexicon.rst
@@ -275,6 +275,9 @@
 **RPO**
     Reverse postorder
 
+**RTTI**
+    Run-time Type Information
+
 S
 -