1. The margins of something, such as property.
2. A different type of business or goods than one principally engages in or sells.
3. Mining law. The boundary lines of a mining claim not crossing the vein running on each side of it. — Also written sidelines. Cf. END LINES. [Cases: Mines and Minerals 18. C.J.S. Mines and Minerals §§ 48–49.]