a rule that determines when a player is in an illegal position on the field:

• Closer to the opponent’s goal than both the ball and the second-last opponent (usually the last defender).
• Actively involved in the play (e.g., receiving the ball or interfering with an opponent).

In football, an “offside” is when an attacking player is ahead of the ball and the second-to-last defender when the ball is played to them.