1. The manner of thinking, behaving, or reacting which is characteristic of a specific person, such as a nervous disposition: Janet had a very optimistic temperament, or mood, and sang while she was in the shower after she had slept well and it was the weekend!
2. Excessive irritability or sensitiveness: Henry was an actor with extreme irritability or temperament because he resented any suggestions from the director.
3. According to medieval physiology, the physical and mental mannerisms or personalities of a person are caused by one of the four humors: In her history class concerning the Middle Ages, Sharon learned about the temperament of people’s behavior being dominated by or issuing from their normal bodily functions in relationship to blood, phlegm, yellow bile and black bile.
4. Etymology: existing since about 1412, "proportioned mixture of elements", from Latin temperamentum, "proper mixture"; from temperare, "to mix".

In medieval theory, it meant a combination of qualities (hot, cold, moist, dry) that determined the nature of an organism; this was extended to a combination of the four humors (sanguine, choleric, phlegmatic, and melancholic) that made up a person's characteristic disposition.

The general sense of "habit of mind, natural disposition" is from 1821; then temperamental, "of or pertaining to temperament" appeared in about 1646; and in the sense of "moody" it is recorded from about 1907.