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"). Quiddity is often compared to the haecceity or "thisness" of an item.

Also used in law to refer to a quibble or academic point. An example can be seen in Hamlet's graveside speech found in Hamlet by William Shakespeare. "Where be his quiddities now, his quillets, his cases, his tenures" says Hamlet referring to a lawyer's quiddities.

Quiddity is also the mystical dream sea in Clive Barker's The Great and Secret Show that exists as a higher plane of human existence. It is featured as more of a literal sea in the novel's sequel, Everville.