a line at the side of something.

a business or activity pursued in addition to one's primary business; a second occupation.

an additional or auxiliary line of goods

a grocery store with a sideline of household furnishings.

Sports .

either of the two lines defining the side boundaries of a field or court.

sidelines, the area immediately beyond either sideline, where the substitute players sit.

sidelines, the position or point of view taken by a person who observes an activity or situation but does not directly participate in it.

to render incapable of participation, esp. in anything involving vigorous, physical action, as a sport

An injury to his throwing arm sidelined the quarterback for two weeks.

noun

distraction , diversion , hobby , leisure activity , leisure pursuit , moonlighting , recreation , second job , side job , side project , subsidiary , avocation , bench , boundary , interest