What is "consumer's surplus"

Consumer's surplus \Consumer's surplus\ (Polit. econ.) The excess that a purchaser would be willing to pay for a commodity over that he does pay, rather than go without the commodity; -- called also

consumer's rent.

The price which a person pays for a thing can never exceed, and seldom comes up to, that which he would be willing to pay rather than go without it. . . . The excess of the price which he would be willing to pay rather than go without it, over that which he actually does pay, is the economic measure of this surplus satisfaction. It has some analogies to a rent; but is perhaps best called simply consumer's surplus.
--Alfred Marshall.