codegen unit is aka CGU (#459)

Author: tshepang

src/appendix/glossary.md

@@ -10,7 +10,7 @@ AST                     |  the abstract syntax tree produced by the syntax crate
 binder                  |  a "binder" is a place where a variable or type is declared; for example, the `<T>` is a binder for the generic type parameter `T` in `fn foo<T>(..)`, and \|`a`\|` ...` is a binder for the parameter `a`. See [the background chapter for more](./background.html#free-vs-bound)
 bound variable          |  a "bound variable" is one that is declared within an expression/term. For example, the variable `a` is bound within the closure expression \|`a`\|` a * 2`. See [the background chapter for more](./background.html#free-vs-bound)
 codegen                 |  the code to translate MIR into LLVM IR.
-codegen unit            |  when we produce LLVM IR, we group the Rust code into a number of codegen units. Each of these units is processed by LLVM independently from one another, enabling parallelism. They are also the unit of incremental re-use.
+codegen unit            |  when we produce LLVM IR, we group the Rust code into a number of codegen units (sometimes abbreviated as CGUs). Each of these units is processed by LLVM independently from one another, enabling parallelism. They are also the unit of incremental re-use.
 completeness            |  completeness is a technical term in type theory. Completeness means that every type-safe program also type-checks. Having both soundness and completeness is very hard, and usually soundness is more important. (see "soundness").
 control-flow graph      |  a representation of the control-flow of a program; see [the background chapter for more](./background.html#cfg)
 CTFE                    |  Compile-Time Function Evaluation. This is the ability of the compiler to evaluate `const fn`s at compile time. This is part of the compiler's constant evaluation system. ([see more](../const-eval.html))