diff --git a/reference/src/glossary.md b/reference/src/glossary.md
index 2e70d7bf..b5b4769c 100644
--- a/reference/src/glossary.md
+++ b/reference/src/glossary.md
@@ -160,6 +160,15 @@ For unsafe code, however, the burden is still on the programmer.
 
 Also see: [Soundness][soundness].
 
+#### Unspecified behavior
+[unspecified]: #unspecified
+
+*Unspecified behavior* is not an error condition in the abstract machine, but beyond that, the Rust language provides no other guarantees about what behavior these programs have.
+
+For example, the field offsets of `repr(Rust)` types is *unspecified*, so a Rust program that prints the offset of a field exhibits *unspecified behavior*: it prints something, but we do not make any guarantees about what it prints. What it prints can therefore change across compiler versions, depending on compiler flags, or even across compiler invocations.
+
+Programs that make assumptions about what a particular source of *unspecified behavior* does often end up exhibiting *undefined behavior* when those assumptions are incorrect. For example, making the assumption that a field of a `repr(Rust)` struct is at a particular offset might lead a program to exhibit *undefined behavior* if that assumption is incorrect.
+
 #### Soundness (of code / of a library)
 [soundness]: #soundness-of-code--of-a-library