The fifth board card, and the final betting round.
Also called fifth street, and with no more cards coming, every draw has either hit or missed. Example. With J♠ 9♠ on K♠ 8♦ 3♠ 2♣, a 5♠ on the river completes the flush and a 7♦ misses. A missed draw is as good as having nothing. When river betting ends, you go to showdown.