A geometric figure with three sides and three angles is called a triangle.
The word triangle comes from Latin roots meaning “three” and “angle.” In elementary geometry, a triangle is a polygon made from three straight line segments joined end to end. Those segments are its sides, and their three meeting points are its vertices.
Triangles are classified in two main ways. By side length, they may be equilateral, isosceles, or scalene. By angle size, they may be acute, right, or obtuse. Every ordinary Euclidean triangle has interior angles that add up to 180 degrees.
The main trap is confusing a triangle with any three-sided shape in a non-Euclidean or curved setting. In standard school geometry, however, the definition is clear: three straight sides and three interior angles. A circle has no straight sides, while rectangles and squares each have four sides.