A downcard is a card which is dealt to a player face down and is kept hidden from all other players. Downcards are what provide the opportunity for strategy and bluffing in poker; otherwise it would merely be a game of chance like War.
Different forms of poker have different numbers of downcards. In Hold 'em, each player is dealt two downcards to start the game, and then betting and the revealing of the community upcards ensues. In 7-Stud, each player is dealt two downcards and one upcard to start the game, but is dealt a third downcard as the last of the seven cards. In five-card draw, each player is dealt five downcards to start the game, and may be dealt further downcards later.