Full Idea
In first-order predicate calculus validity is defined thus: an argument is valid iff every model that makes the premises of the argument true also makes the conclusion of the argument true.
Clarification
'Iff' means if and only if
Gist of Idea
If every model that makes premises true also makes conclusion true, the argument is valid
Source
Joseph Melia (Modality [2003], Ch.2)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.48
A Reaction
See Melia Ch. 2 for an explanation of a 'model'. Traditional views of validity tend to say that if the premises are true the conclusion has to be true (necessarily), but this introduces the modal term 'necessarily', which is controversial.