Full Idea
An essential attribute of something is an attribute it shares with all its counterparts.
Gist of Idea
Essential attributes are those shared with all the counterparts
Source
David Lewis (Counterpart theory and Quant. Modal Logic [1968], III)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.35
A Reaction
I don't like this. It ties essence entirely to identity, but I think essence precedes identity. Essence is a nexus of causal and explanatory powers which bestows an identity on each thing. But essence might be unstable, and identity with it.