It's a plane figure bounded by 3 straight lines that intersect at 3 vertices
[DOC] [TLE] Geometry basic concepts Part 1, points lines and triangles | SureSolv [PAR] May 20, 2016 ... On a piece of paper or on a plane you are at liberty to place the origin at ... Any 
shape bounded by only straight lines consists of more than two lines (at least 3) 
and the meeting points of these lines are called vertices.