River is a casino term used in multiple variations of poker that refers to the last community card dealt by the dealer.
A hand of poker is played in different stages, and each stage starts after the previous round of betting has finished. In Texas Hold’em, for example, the dealer will first deal three cards face-up, and this stage is called the Flop. After another round of betting, the dealer will deal the next card called the Turn. Finally, the dealer will deal the last community card, and that card/stage is called the River.
In other variations of poker, like seven-card stud, there are five rounds of betting in total. Due to the time it takes to get to the last card, staying until the final round of betting is creatively called going to the river, which explains where the term comes from.