It refers to the actual amount that goods or services transacted from sellers to buyers in the market.
Under equilibrium, where quantity demanded equal to quantity supplied, the quantity transacted equals the quantity demanded and quantity supplied.
If the market is not in the equilibrium:
1. quantity demanded is smaller than quantity supplied, then the quantity transacted is equal to the quantity demanded;
2. quantity supplied is smaller than quantity demanded, then the quantity transacted is equal to the quantity supplied.
(refer to shortage and surplus)