Full Idea
According to Fine's definitional characterization of essential properties, they are those of an object's properties that are part of the object's 'definition'.
Gist of Idea
Essential properties are part of an object's 'definition'
Source
report of Kit Fine (Essence and Modality [1994]) by Adolph Rami - Essential vs Accidental Properties §2
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.6
A Reaction
This demands not just an account of what a definition is, but also the notion that there is only one fixed and correct definition (since the object presumably only has one essence) - but there seems to be something relative about a good definition.