The noun brevity means shortness or conciseness. If you give a report on agriculture in the northern hemisphere in 3 minutes, you have done it with incredible brevity.
Brevity comes from brevis, which means "brief" in Latin. You can use brevity for things that are literally short, like the brevity of an elevator meeting, or the quality of fleetingness, like the brevity of youth. Brevity is also a style of speaking or writing with economy. "Brevity is the soul of wit," remarks Polonius in Shakespeare's Hamlet, suggesting that witty people know how to make a point without unnecessary words.