Term
Sideline
The lines along the left and right sides of the court.
The lines along the left and right sides of the court, meeting the net and the baseline at right angles (3.A.4.b). The sideline is part of the court, so a ball that touches it is in (Section 2, Definitions, 8.B, A ball that touches the line is in).
See it in a figure
This figure is from the rule "Court size and lines".
How to read the sources: numbers like (8.F.4) are rulebook sections, and links are outside sources. An "Author's note" is the author's personal experience, not an official rule.