Full Idea
E is an essence if and only if (a) 'has E essentially' is instantiated in some world or other, and (b) for any world W and property P, E entails 'has P in W' or 'does not have P in W'.
Gist of Idea
Essences are instantiated, and are what entails a thing's properties and lack of properties
Source
Alvin Plantinga (World and Essence [1970], IV)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.69
A Reaction
'Entail' strikes me as a very odd word when you are talking about the structure of the physical world (or are we??). Why would a unique self-identity (his candidate for essence) do the necessary entailing?