In poker, the showdown is the situation when, if more than one player remains after the final betting round, the remaining players expose and compare their hands to determine the winner or winners.
Robert's Rules of Poker state that the last player to take aggressive action by a bet or raise is the first to show the hand—unless everyone checks (or is all-in) on the last round of betting, then the first player to the left of the dealer button is the first to show the hand.
[1] To speed up the game, a player holding a probable winner is encouraged to show the hand without delay.
[1] There has been a recent trend in public cardroom rules to limit the ability of players to request to see mucked losing hands at the showdown.
Specifically, some cardrooms only grant the right to view a mucked losing hand if the requesting player articulates a concern about possible collusion.