For the metallurgical process, please see Tempering.
A temper was a tendency of becoming really angry.
In 2268, while talking to Captain James T. Kirk, Parmen said, "I am sure that you, too, have been out of sorts and have been driven to fits of temper and rage." (TOS: "Plato's Stepchildren")
In 2373, B'Elanna Torres expressed to Harry Kim that she had a temper, but that didn't mean that she was always hostile. (VOY: "Displaced")