The quantity exchanged between buyers and sellers when a market is in equilibrium. The equilibrium quantity is simultaneously equal to both the quantity demanded and quantity supplied, which means that there is no shortage nor surplus in the market. This is, in fact, the prime criterion for market equilibrium. If buyers are able to buy all of the good they're willing and able to buy (no shortage) and sellers are able to sell all of the good they're willing and able to sell (no surplus), then neither side of the market is inclined to change the existing terms of trade. And that's equilibrium.