In poker, a tell is a detectable change in a player's behavior that gives clues to that player's hand. Possible tells include leaning forward or back, placing chips with more or less force, fidgeting, changes in breathing or tone of voice, direction of gaze and actions with the cards, cigarettes, or drinks.
For example, a player with a weak hand, hoping to bluff, may throw his chips into the pot forcefully and with a direct gaze at a player he hopes to discourage from calling.
Tells may be common to a class of players or unique to a single player. A player gains an advantage if she observes another player's tell, particularly if that action is unconscious and reliable. However, better players may fake tells, hoping to lead their opponents into costly traps when they rely on the false information. So the observing, creating, and evaluating of tells can add another level to the play of poker.