Definitions can be classified into two large categories, intensional definitions (which try to give the essence of a term) and extensional definitions (which proceed by listing the objects that a term describes).Another important category of definitions is the class of ostensive definitions, which convey the meaning of a term by pointing out examples.It is a list naming every object that is a member of a specific set.
In the definition "An elephant is a large gray animal native to Asia and Africa", the word "elephant" is the definiendum, and everything after the word "is" is the definiens.
(Early modern philosophers like Locke used the corresponding English terms "nominal essence" and "real essence").
The name "hobbit", for example, is perfectly meaningful. But one could not know the real nature of hobbits, even if there were such things, and so the real nature or quid rei of hobbits cannot be known.
he says that the meaning of a made-up name can be known (he gives the example "goat stag"), without knowing what he calls the "essential nature" of the thing that the name would denote, if there were such a thing.
This led medieval logicians to distinguish between what they called the quid nominis or "whatness of the name", and the underlying nature common to all the things it names, which they called the quid rei or "whatness of the thing".