Geometric

The distance from the center of a regular polygon to the midpoint of one side.

The length from the center of a regular polygon to the center of one of its sides. If you look close, it is equivalent to the height of a triangle if you were to slice the polygon into the three-sided shapes.

The point on a segment that lies exactly halfway from each end of the segment. The distance from the endpoint of a segment to its midpoint is half the length of the whole segment.

a line segment connecting two points on a circle; not "do re mi"

A line segment whose endpoints are both on a circle. Not a collection of musical notes.

Any segment from one point on a circle to another. Despite what you might think, they aren't all that musical.

the bottom part of fractions; in this case when more than one fraction has the same bottom as all the others

A unit of measure for the size of an angle. One full rotation is equal to 360 degrees. A right angle is 90 degrees. One degree equals radians.

Pairs of angles formed where two lines intersect. These angles are formed by rays pointing in opposite directions, and they are congruent. Vertical angles come in pairs.

angles that share a side

Two angles that share both a side and a vertex. They stick by each other through and through.

Angles that add up to 90°

Two wrongs don't make a right, but two complementary angles do. They're two angles that add up to 90° exactly.

Two angles that add up to 90°. Two wrongs don't make a right, but two complementary angles sure do.

A ray, one of a pair, that shares a common vertex with an angle, lies within the interior of that angle, and creates, with its partner, three new angles of equal measure. Angle trisectors come in pairs.

A ray that shares a common vertex with an angle, lies within the interior of that angle, and creates two new angles of equal measure.