Define "interior mutability"

shepmaster · web-flow · commit c8886dcba6fd · 2018-10-07T12:02:40.000-04:00
diff --git a/reference/src/introduction.md b/reference/src/introduction.md
@@ -3,6 +3,8 @@
 
 ## Terms
 
+* *interior mutabilitiy* — mutating memory where there also exists a live shared reference non-transitively pointing to the same memory.
+
 (TODO: definitions)
 
 * *niche*
