A card game created in 1971 and currently distributed by Mattel, based on the game Crazy Eights.
In the game, players take turns discarding cards to be the first to have all their cards discarded from their hand. Certain action cards call the next color to play, reverse the direction of play, force the next player to draw a certain number of cards, or forfeit the next player's turn altogether. A player has to shout "Uno" (Spanish/Italian for "one") to warn other players they are down to their last card; a round ends when a player runs out of cards.
One popular house rule (not an official rule) is to counter a "Draw 2/4" card by playing a similar "Draw 2/4" card, thus "stacking" the number of cards the following player has to draw.
