essence

In philosophy, all that makes a thing what it is and is indispensable to the thing. Philosophers have often distinguished nominal essences from real essences. A nominal essence is a group of terms used to define a concept: thus, the nominal essence of the concept of a horse could be ‘anything that neighs and has a mane and four legs’. A real essence is either a group of universals objectively given in nature (this is also called a form) or (as in the work of John Locke) the underlying structure of an object; for example, its atomic structure.