A line that marks the boundary of a playing field or court. The player stepped out of bounds when he crossed the sideline.
An activity done in addition to one's main job, especially to earn extra income. She started a small baking business as a sideline to her teaching career.
A position or point of view that is not central or directly involved in an activity. He watched the debate from the sideline, not wanting to get involved in the argument.
The area along the side of a sports field where coaches, substitutes, and other team personnel stand during play. The trainer sprinted from the sideline to help the injured player.