A semicircle (180°) is that part of a circle which is contained between the diameter, and half the circumference.
A quadrant is the fourth part of a circle, being contained between two radii, and an arc of 90°.
A segment is that part of a circle which is cut off by a chord.
A sector is that part of a circle contained between two radii, and an arc.
A secant is a line which cuts a circle, lying partly within, and partly without it.
A tangent is a line which touches a circle, or curve, without cutting it.
The point of contact is where a tangent touches an arc.
Triangles are figures having three sides, and three angles.
An equilateral triangle has its three sides equal.