The definition of congruent. Congruent lines, angles shapes.
Math Open Reference

Points of concurrency  the point where three or more lines intersect
Math Open Reference

Definition and properties of concentric circles
Math Open Reference

Definition of 'Perpendicular Bisector'
Math Open Reference

Definition of 'Line Bisector' and a general discussion of bisection. Link to 'angle bisector'
Math Open Reference

Definition of 'base' and the various different ways the word is used in geometry
Math Open Reference

Adjacent sides definition: Two lines that meet at a polygon vertex.
Math Open Reference
