What is Sideline?

Sideline definition and meaning on Dictionary terms:
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.

verb (used with object), side