Real Dictionary

sideline
If someone or something is sidelined, they are made to seem unimportant and not included in what people are doing.
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.
countable noun: A sideline is something that you do in addition to your main job in order to earn extra money.
plural noun: The sidelines are the lines marking the long sides of the playing area, for example on a football field or tennis court.

sideline in American English
a line at or along the side
to remove from active participation
noun: a line at the side of something

sideline in British English
to prevent (a player) from taking part in a game
noun: a line that marks the side boundary of a playing area

sideline in Retail
noun: A sideline is a business activity carried on at the same time, by the same person, or at the same premises, as another, more important, business activity.