Term used for the theory, going back to Gottfried Wilhelm Leibniz (1646-1716), of 'possible worlds'; used to analyze necessity and possibility and similar notions, which are known as modal notions.
The actual world is regarded as merely one among an infinite set of logically possible worlds, some nearer to the actual world and some more remote. A statement is called necessary if it is true in all possible worlds, and possible if it is true in at least one. Possible worlds are usually regarded as real but abstract possibilities.
However, for David Kellogg Lewis they are concrete worlds, like this one only without any spatial or temporal connections with it.
D K Lewis, On the Plurality of Worlds (1986)
Also see:· counterpart theory