A shape with three sides is called a triangle. Triangles are a fundamental geometric shape characterized by three edges and three vertices. They come in various types based on their angles and side lengths. For example, an equilateral triangle has all three sides of equal length and all angles measuring 60 degrees. An isosceles triangle has two sides of equal length and two equal angles, while a scalene triangle has all sides of different lengths and all angles differing. Triangles are essential in geometry and are foundational in many areas of mathematics, architecture, and engineering due to their strength and stability.