Full Idea
To say that an object a has a property P essentially is to say that it has P, and in all of certain worlds (all possible, all in which something identical with it exists, ...) the object identical with it has P. This is the standard de re interpretation.
Gist of Idea
De re essentialism standardly says all possible objects identical with a have a's essential properties
Source
Baruch Brody (Identity and Essence [1980], 5.4)
Book Reference
Brody,Baruch: 'Identity and Essence' [Princeton 1980], p.103
A Reaction
This view always has to be qualified by excluding trivially necessary properties, but that exclusion shows clearly that the notion of essential is more concerned with non-triviality than it is with necessity.