In geometry, a side refers to a straight line segment that forms part of the boundary of a two-dimensional shape, such as a triangle, square, or polygon. The number of sides a shape has depends on its type. For example, a triangle has three sides, a square has four sides, and a pentagon has five sides.

Sides are important because they define the shape and structure of an object. They also play a key role in determining the perimeter of a shape, which is the total length of all its sides.

In terms of naming the sides of a shape, they are typically labeled using lowercase letters or by using the vertices of the shape. For example, in a triangle with vertices A, B, and C, the sides can be denoted as AB, BC, and AC.

Understanding the concept of sides is fundamental in geometry and helps us analyze and classify different shapes based on their properties.