Full Idea
On my view, necessity is the truth of a sentence in every member of a set of possible worlds, together with its nonfalsity in every other possible worlds.
Gist of Idea
A sentence is necessary if it is true in a set of worlds, and nonfalse in the other worlds
Source
Robert Hanna (Rationality and Logic [2006], 6.6)
Book Reference
Hanna,Robert: 'Rationality and Logic' [MIT 2006], p.196