What is the definition of a triangle?

A triangle is defined as a three-sided polygon, which means it is a closed geometric figure with three straight sides and three vertices (corners). This shape is fundamental in geometry and is characterized by its angles and side lengths.

The definition encompasses several important properties, including the fact that the sum of the interior angles in a triangle always equals 180 degrees, regardless of the type of triangle (right, acute, or obtuse). Additionally, the triangle can be classified based on the lengths of its sides (such as equilateral, isosceles, or scalene) or the measures of its angles, offering a wide range of applications in various fields like engineering, architecture, and art.

The other options do not accurately describe what a triangle is. A three-dimensional shape refers to any object that possesses height, width, and depth, which is not applicable to the flat, two-dimensional nature of a triangle. A four-sided polygon describes a quadrilateral, such as a square or rectangle, and a point in geometric space refers to a specific location with no dimensions, neither of which relates to the definition of a triangle.