Full Idea
If necessity is explained in terms of possible worlds, ...then an essential property is a property that a thing has in all possible worlds in which it exists.
Gist of Idea
An essential property is one had in all the possible worlds where a thing exists
Source
Robert C. Stalnaker (Anti-essentialism [1979], p.71)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.71
A Reaction
This seems to me to be a quite shocking confusion of necessary properties with essential properties. The point is that utterly trivial properties can be necessary, but in no way part of the real essence of something.