Full Idea
A possible world is just a maximal possible state of affairs.
Gist of Idea
A possible world is a maximal possible state of affairs
Source
Alvin Plantinga (Transworld Identity or worldbound Individuals? [1973], I)
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.147
A Reaction
The key point here is that Plantinga includes the word 'possible' in his definition. Possibility defines the worlds, and so worlds cannot be used on their own to define possibility.