A sideline is something that you do in addition to your main job in order to earn extra money.

The sidelines are the lines marking the long sides of the playing area, for example on a football field or tennis court.

If you are on the sidelines in a situation, you do not influence events at all, either because you have chosen not to be involved, or because other people have not involved you.

If someone or something is sidelined, they are made to seem unimportant and not included in what people are doing.

In sports, if a player is sidelined, he or she is prevented from playing for a period of time, for example because of an injury.