When studying Geometry we use: Undefined terms
... line segment, ray, angle, congruent, acute angle, right angle, obtuse angle, straight angle, polygon, triangle, adjacent angles, union, intersection, collinear, theorem, midpoint, trisection points, angle bisector, angle trisectors, and postulate. ...
... line segment, ray, angle, congruent, acute angle, right angle, obtuse angle, straight angle, polygon, triangle, adjacent angles, union, intersection, collinear, theorem, midpoint, trisection points, angle bisector, angle trisectors, and postulate. ...