the closed figure (including its internal area) composed of three non-collinear points on the plane and the line segments connecting each two points is called a triangle. each line segment that makes up a triangle is called a side of the triangle. the common endpoints of two adjacent sides are called the vertices of a triangle. the angle between two adjacent sides is called the interior angle of a triangle, or simply the angle of a triangle. the angle between the extension of one side of a triangle and its adjacent side is called the exterior angle of the triangle. the triangle with a, b, and c as vertices is recorded as: △abc, and pronounced as: triangle abc.

a figure enclosed by three straight lines on a plane or three arcs on a sphere is called a plane triangle; a figure enclosed by three arcs is called a spherical triangle, also called a trigonal shape. △abc..