Something that belongs to a thing such that without it the thing cannot be the
thing it is; necessary characteristic or property
of a thing.
Whether there really are
essences of things is a moot question, but it is certainly true that there
are at least verbally necessary characteristics that are involved in the
definition of a term.
Thus, something cannot be a crow without being a bird, and in that sense it
belongs to a crow's essence to be a bird, and this is shown by a proper
definition of the term 'crow' that should include that anything correctly
called a crow also is correctly called a bird.
Often far more is meant, implied or suggested by the term 'essence', and
usually the problem is that this involves problems of modal logic, such as in
what sense the supposed necessary features of something are necessary.