A triangle is a geometrical shape that is formed by three straight lines or line segments that connect three non-collinear points in a plane. These three points are called vertices of the triangle, and the line segments connecting them are called sides. The triangle is a closed figure, and the region enclosed by the sides is called the interior of the triangle.

A triangle can be classified according to the length of its sides and the measurement of its angles. For example, if all three sides of a triangle have equal length, it is called an equilateral triangle. If two sides of a triangle have equal length, it is called an isosceles triangle. If none of the sides have the same length, it is called a scalene triangle.

Similarly, triangles can be classified by the measurement of their angles. For example, if all three angles of a triangle are acute (less than 90 degrees), it is called an acute triangle. If one angle of a triangle is a right angle (90 degrees), it is called a right triangle. If one angle of a triangle is obtuse (greater than 90 degrees), it is called an obtuse triangle.