What is the definition of a triangle?

A triangle is defined as a three-sided polygon. This geometric shape consists of three straight sides and three angles. The sum of the interior angles in a triangle always equals 180 degrees. Triangles can vary in type based on their side lengths and angle measurements; for instance, they can be classified as equilateral, isosceles, or scalene based on their sides, and as acute, right, or obtuse based on their angles. Understanding this definition is fundamental in geometry, as triangles are one of the simplest forms of polygons, and they serve as the building blocks for more complex geometric shapes.