The poker term river is used in community card games to refer to the last card dealt on the board. The river can refer to the actual card or the last betting round that takes place when that card is dealt.
In Texas Hold’em, the river is the fifth and last community card, following the fourth card, called the turn, and the first three cards, which are called the flop.
For example, let’s say you are holding As5s, and the board runs out as 2c 8s Js Kd, where you decide to donk bet and take down the pot. You ask the dealer to go rabbit hunting and see that river would have been Qs, in which case, you would have hit your flush on the last card on the river and had the nuts.