"Side" is a term that is used in both geometry and algebra.
Sides of a polygon
The sides of a polygon are the three or more line segments that form the polygon.
Each line segment (AB, BC, AC) constitutes one side of the polygon, which in this case, is a triangle.
Sides of an angle
Each ray in the angle shown is one side of the angle.
Sides of an equation
The sides of an equation are the expressions on either side of the equals (=) sign.
4 + n = 7 - 1
4 + n is the left side of the equation.
7 - 1 is the right side.