Offside

When a ball is passed to a player behind the line of defense.

If the player was behind said line before the ball left the other players foot and the first player ran past the line to get to the ball that would be onside

In European Football (soccer)

When Jarrod Bowen passed the ball to Lucas Paqueta ot was ruled offside as he ran past the line of defense before the ball left Bowen's foot.

offside rule

Something every guy will ask you to define when a girl will tell them they like football.

Amanda: i like football too!!
Bryton, willy and chad in unison: OH YEAH? tell us what the offside rule is then...
Amanda: leaves