Essence is the ultimate character of a thing, it is that which makes a thing what it is. Appearance is essential to get to the essence. The essence is inexhaustible and irreducible to its existent form.
Essence or the essential, is opposed to the inessential. Another word for essence, is core, or the necessary. Essence is usually conceived of as the logical inner as opposed to the outer, yet we access that inner from the outer. Just as everything else does, the essence has its own process of historical development, is not eternal, but in a process of change.