Students will learn how to discern between proper and improper definitions, including parallel, perpendicular, angle, etc.
Geometric definitions example by Khan Academy
Geometric definitions by Khan Academy
Conclude by giving your students these challenges:
HSG.CO.A.1: Know precise definitions of angle, circle, perpendicular line, parallel line, and line segment, based on the undefined notions of point, line, distance along a line, and distance around a circular arc.