Full Idea
Completeness is when, if a sentences holds in every model of a theory, then it is logically derivable from that theory.
Gist of Idea
If a sentence holds in every model of a theory, then it is logically derivable from the theory
Source
Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V)
Book Reference
Feferman,S/Feferman,A.B.: 'Alfred Tarski: life and logic' [CUP 2008], p.281