Full Idea
The equation of 'necessity' with 'true in every possible world' is known as the S5 conception, corresponding to the strongest of C.I.Lewis's five modal systems.
Gist of Idea
Equating necessity with truth in every possible world is the S5 conception of necessity
Source
Stephen Read (Thinking About Logic [1995], Ch.4)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.118
A Reaction
Are the worlds naturally, or metaphysically, or logically possible?