A circle is the set of all points that have the same distance from another point (the center of the circle). This distance is called radius.

Parts of a circle

In the following we name the different parts of a circle. Consider the following image:

circle

Center
the center point of the circle.
Radius
any line going from the center point to any point on the circle.
Diameter
a line going from one point on the circle to another point on the circle passing through the center point (the length is twice that of the radius).
Chord
a line going from one point on the circle to another point (the diameter is also a chord).
Secant
any line that contains a chord.
Tangent
a line touching the circle in one point.
Point of tangency
the point where the tangent is touching the circle.