1. a radius of a circle or sphere is any of the line segments from its center to its perimeter.
2. a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle.
3. The chord of a circle is a line segment joining any two points on the circle.
4. a secant of a curve is a line that intersects the curve at a minimum of two distinct points.
5. the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point.
6. a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, π radians, or a half-turn). It has only one line of symmetry (reflection symmetry).
7. A minor arc is an arc smaller than a semicircle. A central angle which is subtended by a minor arc has a measure less than 180°.
8. A major arc is an arc larger than a semicircle. A central angle which is subtended by a major arc has a measure greater than 180°.
9. A central angle is an angle with its vertex at the center of a circle, with its sides containing two radii of the circle.
10. An inscribed angle is an angle whose vertex lies on a circle and its two sides are chords of the same circle.
-E.F.
