In philosophy quiddity is identity or "whatness", i.e. something's "what it is".
The term has been used in the work of Aristotle in reference to the effects of entities' aspects: "matter" and "form". According to his explanation, all entities have two aspects, "matter" and "form". It is the particular form imposed that gives some matter its identity, its quiddity or "whatness" (i.e., its "what it is").
Also used in law to refer to a quibble or academic point. An example is in Hamlet's speech by the grave side in Hamlet by William Shakespeare, where he refers to a lawyer's quiddities: "Where be his quiddities now, his quillets, his cases, his tenures."