Full Idea
For Lewis, if a property possessed by a given individual or kind is missing in some of the contextually relevant counterparts, that property is accidental to the individual or kind; if it is possessed by all of them, that property is essential.
Gist of Idea
An essential property is one possessed by all counterparts
Source
report of David Lewis (On the Plurality of Worlds [1986], 248-263) by Crawford L. Elder - Real Natures and Familiar Objects 1.4
Book Reference
Elder,Crawford L.: 'Real Natures and Familiar Objects' [MIT 2004], p.18
A Reaction
This is a sophisticated version of the idea that essential properties are just necessary properties. It might make sense with a very sparse view of properties (mainly causal ones), but I think of essences as quite different from necessities.