What is the common term for a 3-sided polygon?

A 3-sided polygon is commonly known as a triangle. This term is derived from the prefix "tri-" which means three, and "angle," referring to the three angles formed at the vertices of the shape. In geometry, triangles are significant because they are the simplest form of polygon and possess unique properties, such as the fact that the sum of the interior angles of a triangle always equals 180 degrees.

In contrast, a quadrilateral, which has four sides, hexagon, which has six, and pentagon, which has five, do not apply to the description of a 3-sided polygon. Each of these shapes is referenced by the prefix that indicates the number of their sides. Thus, the term for a three-sided figure is correctly identified as a triangle.