What indicates that a forward pass has occurred?

A forward pass in American football is defined by the direction in which the ball is thrown relative to the line of scrimmage. When a player throws the ball towards the opponent's end line, it signifies that the ball is being moved in a forward direction, which is the fundamental characteristic of a forward pass. This movement toward the opposing team's end zone differentiates it from other types of passes, such as lateral or backward passes, which do not advance the ball in a forward manner.

The other options do not accurately convey the definition of a forward pass. While throwing the ball to a teammate may lead to a pass, it does not specify the direction; the receiver's position does not determine if a pass is forward; and the quarterback stepping back is a common action but is not a definitive indicator that a forward pass has taken place. Thus, the indication of a forward pass is clearly defined by the ball being thrown toward the opponent's end line.