[llvm] [LLVM][Docs] Define IR in the lexicon (PR #221388)

[Prajit Rahul via llvm-commits]

https://github.com/Prajit-Rahul created https://github.com/llvm/llvm-project/pull/221388

Define IR in the LLVM lexicon using terminology from the LLVM Language Reference Manual, including its SSA-based structure and three equivalent representations.

Fixes #139867

>From 915a186de37a0890da2e72e14bafbb024fa68085 Mon Sep 17 00:00:00 2001
From: Prajit-Rahul <prajitrahul05 at gmail.com>
Date: Sat, 5 Sep 2026 04:12:14 +0000
Subject: [PATCH] [LLVM][Docs] Define IR in the lexicon

---
 llvm/docs/Lexicon.md | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/llvm/docs/Lexicon.md b/llvm/docs/Lexicon.md
index 022e9b5811f38..eb429848dddcc 100644
--- a/llvm/docs/Lexicon.md
+++ b/llvm/docs/Lexicon.md
@@ -162,6 +162,13 @@ This document is a work in progress!
 :   Inter-Procedural Optimization. Refers to any variety of code optimization
     that occurs between procedures, functions or compilation units (modules).
 
+**IR**
+:   Intermediate Representation. LLVM IR is the common code representation
+    used throughout LLVM's compilation process. It is typed and based on
+    Static Single Assignment (SSA), with three equivalent forms: in-memory
+    IR, on-disk bitcode, and human-readable assembly. See the
+    {doc}`LLVM Language Reference Manual <LangRef>`.
+
 **ISel**
 :   Instruction Selection