Full Idea
Essentialism, as I shall understand it, is the doctrine that among the attributes of a thing some are essential, others merely accidental. Its essential attributes are those it has necessarily, those it could not have lacked.
Gist of Idea
Essentialism says some of a thing's properties are necessary, and could not be absent
Source
Richard Cartwright (Some Remarks on Essentialism [1968], p.149)
Book Reference
Cartwright,Richard: 'Philosophical Essays' [MIT 1987], p.149
A Reaction
The problem with this, which Cartwright does not address, is that trivial and gerrymandered properties (such as having self-identity, or being 'such that 2+2=4') seem to be necessarily, but don't seem to constitute the essence of a thing.