Definition;When a player is nearest to his opponent's goal line rather than the ball and the second last defender.
A player is being penalised in offside by doing the following;
1.Interfeering with play.
2.Interfeering with an opponent.
3.Gaining advantage by being in that position.
NOT OFFSIDE;
1.Level with the second last defender.
2.Touches the ball from the centre line.
3.Goal kick
4.Throw in
RECEIVING THE BALL FROM OFFSIDE POSITION IS TOTALLY NOT ALLOWED.