Having nothing missing. Completeness is a claim that must be checked, since a set can feel whole and not be.
Latin completus, past participle of complere (to fill up, finish), from com- (thoroughly) + plere (to fill); having nothing left empty, a claim about a container rather than about correctness.
Having nothing missing, which is a claim worth checking rather than trusting on sight — a set can feel full and still have a gap no one noticed.