If recovering the ball in one's opponent's end zone, a player may down the ball by taking a knee.
If a quarterback is running with the ball during his initial possession of same following the snap from the center, he may down the ball by voluntarily sliding from his feet to a sitting or recumbent position.
If a member of the defensive team desires to down a ball in the possession of a member of the offensive team, he may do so by tackling the offensive player, or by merely coming into contact with the offensive player at a time when some part of the offensive player's body other than his feet or hands is touching the ground.
A player in possession of the ball will down the ball if he fumbles it out of bounds.
Down - adj. When referring to a ball: out of play, or dead. When referring to a player who is in possession of the ball: ineligible to advance the ball further, on account of having touched some part of his body other than hands or feet to the ground while in contact with an opposing player.
Down - n. A single period during which a play transpires. It begins with a snap or kickoff or free kick, and ends when the ball or the player in possession of it is declared down by an official, a team scores, or the ball or player in possession of it leaves the field of play. At the end of a play, the ball is dead.
Each possession consists of four downs. If, during a possession, the offensive team advances the ball ten yards or more, they get four more downs within which to try to score, or to at least advance an additional ten yards. If, in four downs, an offensive team has not advanced at least ten yards, then they may elect on the fourth down, to punt, or if they elect to continue to try to advance the ball and fail to gain the necessary yards to accumulate to ten yards for that possession or series of four downs, then possession of the ball reverts to the opposing team at the spot where the ball was downed at the end of the fourth down.
In Canadian football the rules regarding a down are similar. The main difference is that a team gets only three downs to make ten yards.