Full Idea
I conceive of possible worlds as certain sorts of maximal abstract entities according to which certain things (facts, states of affairs) obtain and certain other things do not obtain. They are total ways things might have been.
Gist of Idea
Possible worlds are maximal abstract ways that things might have been
Source
Nathan Salmon (The Logic of What Might Have Been [1989], I)
Book Reference
Salmon,Nathan: 'Metaphysics, Mathematics and Meaning' [OUP 2005], p.131