"side" is referenced in the pages listed below.

Describes how the smallest angle is opposite the shortest side, and the largest angle is opposite the longest side.
Adjacent sides definition: Two lines that meet at a polygon vertex.
Similar triangles - all three sides in same proportion. SSS
The sides of a polygon are defined and two formulas for finding the side length for a regular polygon.
Congruent triangles - all three sides equal. SSS
Included side definition: The common leg of two angles.
Similar triangles - two sides in same proportion, included angle the same. SAS