Definition

A triangle is a simple closed curve composed of three line segments that are joined end to end.
A triangle is the union of three non- collinear points in a plane and the three segments connecting them

The points A, B and C are called the vertices (singular vertex) of the triangle

The segments

are called sides of the triangle.

A triangle is named by stating the three vertices. The above figure shows triangle ABC, written ∆ABC.

The three angles, i.e, ∠ABC, ∠BCA and ∠BAC are called interior angles of ∆ABC.