1. A comic, absurd, or incongruous quality causing amusement.
2. That which is intended to induce laughter or amusement: a writer skilled at crafting humor.
3. The ability to perceive, enjoy, or express what is amusing, comical, incongruous, or absurd.
4. Etymology: from 1340, "fluid" or "juice of an animal or plant", from Anglo-Norman humour, from Old French humor, from Latin umor, "body fluid"; related to umere, "be wet, moist", and to uvescere, "become wet".

In ancient and medieval physiology, "any of the four body fluids" (blood, phlegm, choler, and melancholy or black bile) whose relative proportions were thought to determine the state of people's minds.

This led to a sense of "mood" or "temporary state of mind"; the sense of "amusing quality, funny" is first recorded in 1682, probably by way of the sense of "whim, caprice", which also produced the verb sense of "indulge".