Full Idea
An aim of formal semantics is to define in mathematical terms a set of models such that an argument is valid if and only if it preserves truth in every model in the set, for that will provide us with a precise standard of validity.
Gist of Idea
Formal semantics defines validity as truth preserved in every model
Source
Timothy Williamson (Vagueness [1994], 5.3)
Book Reference
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.149