Full Idea
An object has a property essentially just in case it couldn't conceivably have lacked that property.
Gist of Idea
An object has a property essentially if it couldn't conceivably have lacked it
Source
Alvin Plantinga (De Re and De Dicto [1969], p.35)
Book Reference
Plantinga,Alvin: 'Essays in the Metaphysics of Modality' [OUP 2003], p.35
A Reaction
Making it depend on what we can conceive seems a bit dubious, for someone committed to real essences. The key issue is how narrowly or broadly you interpret the word 'property'. The word 'object' needs a bit of thought, too!