Full Idea
A 'model' of a theory is an assignment of meanings to the symbols of its language which makes all of its axioms come out true.
Gist of Idea
A 'model' is a meaning-assignment which makes all the axioms true
Source
A.George / D.J.Velleman (Philosophies of Mathematics [2002], Ch.7)
Book Reference
George,A/Velleman D.J.: 'Philosophies of Mathematics' [Blackwell 2002], p.193
A Reaction
If the axioms are all true, and the theory is sound, then all of the theorems will also come out true.