In
American football, a thrown ball that does not go forward on the field (see
forward pass for rules on that) -- it may actually travel laterally across the field, or go in any direction between horizontal and straight backwards.
Unlike the forward pass, there are no rules on number of laterals that may be thrown or legal field position from which to throw a lateral. Laterals do not count for passing yards, and usually aren't recorded at all in the game's statistics.
A lateral not caught is a live ball and generally ruled a fumble, as opposed to an uncaught forward pass, which is always ruled incomplete and thus a dead ball. This makes the lateral somewhat more risky than the forward pass.
Some of the most controversial plays in football hinge on whether a thrown ball was a lateral or not. In the 1999-2000 playoffs, the Tennessee Titans' Music City Miracle victory over the Buffalo Bills depended on Frank Wycheck's throw to Kevin Dyson at the Tennessee 20 during the final kickoff return being ruled a lateral. Buffalo fans insist to this day that Wycheck actually threw a forward pass (which would have been illegal), although most replays indicated that the pass travelled straight along the 20, thus laterally.