Down
An offensive team's attempt to advance the ball at least ten yards within four tries.
Each offensive possession is broken into sets of four downs. The offense must advance the ball at least ten yards within those four attempts to earn a new set of downs; failing to do so turns the ball over to the opposing team.
The first three downs are typically used to probe the defense and build toward the needed distance, while fourth down is decision time: the team can go for it, punt to flip field position, or attempt a field goal, depending on distance and location on the field.
Play-by-play commentary often refers to "moving the chains" when a team converts and resets the down count.