In semantics a set of alternative universes that can characterize an agent's beliefs. 
Formalised by certain modal logics.
See http://www.csc.liv.ac.uk/~mjw/pubs/ker95/subsection3_2_3.html

DecisionTrees also are a kind of enumeration of PossibleWorlds - based on a current state if certain events take place and choices are made by an agent the combinations of all possible outcomes are alternate "worlds" that could be manifested. 

In physics some paradoxes of TimeTravel may be resolved by assuming events spawn alternate worlds so that if one goes back in time and kills Hitler as a baby, this TimeLine is not affected but in an alternate universe WII does not occur.