Seventh street is a casino term that refers to the last round of betting, and it is very similar to the term river. The term is used during a game of Seven Card Stud and Stud 8 or Better, both of which are popular variations of poker.
If you are playing Seven Card Stud, during the fifth round of betting, players will have seven cards in their hands, and that is how the term was coined. The dealer will then deal the last community card during the seventh street and if there are multiple players left, the winner will be decided by a showdown that includes players forming the best five-card combinations.
Additionally, in the previous betting stage, players will have six cards in their hands, so you might often hear someone referring to that situation as the sixth street.