Full Idea
A 'centred' possible world is an ordered triple consisting of a possible world, an individual in the domain of that world, and a time.
Gist of Idea
A 'centred' world is an ordered triple of world, individual and time
Source
Robert C. Stalnaker (Conceptual truth and metaphysical necessity [2003], 2)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.205