(Latin: esse, to be)
That which a thing is, or that by which a thing is what it is.
It is also called the quality of a thing because it tells what a thing is.
It is the combination of all the notes or intelligible qualities and only those without which the thing in question cannot be conceived.
In regard to finite things, when the term is used without modification, the specific and metaphysical essence is usually understood, viz., that by which it is the kind of thing it is and different from every other kind of thing.
The specific essence is the complete essence which is found common to all the individuals of any species.
It comprises the generic note, in which it is similar to other species, e.g., the note of animality in the essence of man which is found also in the essence of brute, and the specific difference, namely the perfection which is found only in the members of the particular species, e.g., the perfection of rationality which is peculiar to the species man alone.
Neither of these two notes taken alone can be the expression of a complete essence.
The physical essence of a thing may be defined as the combination of all that is necessarily required for a thing actually to exist, and therefore includes existence and all those perfections which are necessarily connected with specific essence.
New Catholic Dictionary