An offside offense requires all three of the following:
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The player must be in an offside position.
Offside position is when any playable part of a player's body is closer to their opponent's goal than the halfway line, the second-last defender, and the ball.
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At the time the ball is last touched by the player's teammate. And
The offside calculation resets every time a player's teammate touches the ball.
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The player must then become involved in active play.
A player becomes actively involved by playing the ball, interfering with an opponent playing the ball, or gaining an advantage on a goal save.
A player cannot be guilty of an offside offense if they receive the ball directly from a goal kick, corrner kick, or throw-in, or if the player receives the ball from a deliberate play by an opponent, even if the elements above are met.
When an offside offense does occur, it results in an indirect free kick (IFK) for the opposing team at the place where the player became involved in active play.
Law 11 - Full Text
Debunking Misconceptions about Offside
Offside position, by itself, is not an offense
A player can stand in an offside position the entire game if they choose, as long as they do not become involved in active play.
A player does not have to play the ball to be guilty of an offside offense
An offside offense can occur when a player attempts to play the ball, gets in the way of an opponent (including by blocking their vision), or otherwise interferes with an opponent.
Offside restarts are from the location where the player became involved in active play
It does not matter where the player was standing when their teammate last touched the ball. This means that an offside offense can be restarted in the offender's own half of the field if that offender has run back to their half to retrieve the ball.
The restart can also be deep in the offender's attacking half if the player runs a long way from an offside position before getting involved in active play.
A player dribbling the ball cannot become offside, even if they pass all defenders
The ball is likely closer to the goal than the player (i.e., not in an offside position), and the time to judge offside position was when the player's teammate last touched the ball.
It can be legal for a player to gain possession of the ball when they are in an offside position
It is where the player is when their teammate touches the ball, not where the player is when they gain possession of the ball, that matters. So if a player runs past the defense to get to the ball, this is legal.
It is also possible for a player to receive the ball from a pass or mis-kick from an opponent, which would also be legal, even if the player is in an offside position.
Finally, a player in an offside position may legally receive the ball directly from a goal kick, corner kick, or throw-in without being guilty of an offside offense.
It is called "offside," not "offsides"
"Offsides" is an American football term. In soccer, it is called "offside" because the player is temporarily off their side (i.e., off their team).