diff --git a/src/glossary.md b/src/glossary.md
index b7282c3eb..a22c49632 100644
--- a/src/glossary.md
+++ b/src/glossary.md
@@ -7,6 +7,7 @@ Term                    | Meaning
 ------------------------|--------
 AST                     |  the abstract syntax tree produced by the syntax crate; reflects user syntax very closely.
 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.
+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").
 cx                      |  we tend to use "cx" as an abbrevation for context. See also `tcx`, `infcx`, etc.
 DAG                     |  a directed acyclic graph is used during compilation to keep track of dependencies between queries. ([see more](incremental-compilation.html))
 DefId                   |  an index identifying a definition (see `librustc/hir/def_id.rs`). Uniquely identifies a `DefPath`.
@@ -25,6 +26,7 @@ provider                |  the function that executes a query ([see more](query.
 query                   |  perhaps some sub-computation during compilation ([see more](query.html))
 sess                    |  the compiler session, which stores global data used throughout compilation
 side tables             |  because the AST and HIR are immutable once created, we often carry extra information about them in the form of hashtables, indexed by the id of a particular node.
+soundness               |  soundness is a technical term in type theory. Roughly, if a type system is sound, then if a program type-checks, it is type-safe; i.e. I can never (in safe rust) force a value into a variable of the wrong type. (see "completeness").
 span                    |  a location in the user's source code, used for error reporting primarily. These are like a file-name/line-number/column tuple on steroids: they carry a start/end point, and also track macro expansions and compiler desugaring. All while being packed into a few bytes (really, it's an index into a table). See the Span datatype for more.
 substs                  |  the substitutions for a given generic type or item (e.g. the `i32`, `u32` in `HashMap<i32, u32>`)
 tcx                     |  the "typing context", main data structure of the compiler ([see more](ty.html))