Rationale (October, 2015):

CWG felt that the current usage is plain English, not a technical term, and is clear enough.

One would expect to find a definition of the terms “associated class” and “associated namespace” in 6.5.4 [basic.lookup.argdep], but there is none. Note also that “associated class” is used in a different sense in 7.6.10 [expr.eq] bullet 3.6, and that drafting being proposed for other issues also uses the term differently.