2
A flat, two-dimensional shape with straight sides
3
Line connecting two non-adjacent vertices of a polygon
5
A parallelogram with opposite equal acute angles
8
Distance across a circle through its center
10
Straight path extending in two directions without end
11
The amount of space inside the boundary of a flat object
13
Part of a line with two endpoints
14
Three-dimensional figure with circular base and one vertex
15
Solid figure formed by all points equidistant from the center
17
Ratio of the circumference of a circle to its diameter
19
Symmetry around a central point
20
Angle less than 90 degrees