The essence of a thing, or that which differentiates it from other things. Schoolmen say Quid est (what is it?) and the reply is, the Quid is so and so, the What or the nature of the thing is as follows. The latter quid being formed into a barbarous Latin noun becomes Quidditas. Hence Quid est (what is it)? Answer: Talis est quidditas (its essence is as follows).
He knew ... Where entity and quiddity (The ghosts of defunct bodies) fly.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894