Full Idea
Logicians say that a theory T is '(negation) complete' if, for every sentence φ in the language of the theory, either φ or ¬φ is deducible in T's proof system. If this were the case, then truth could be equated with provability.
Gist of Idea
A theory is 'negation complete' if one of its sentences or its negation can always be proved
Source
Peter Smith (Intro to Gödel's Theorems [2007], 01.1)
Book Reference
Smith,Peter: 'An Introduction to Gödel's Theorems' [CUP 2007], p.2
A Reaction
The word 'negation' seems to be a recent addition to the concept. Presumable it might be the case that φ can always be proved, but not ¬φ.