What is it?

Circles are simply closed curves. Where points along the curve converge to close the curve and the results are a circle.

Parts of a Circle:

Chord:
A chord of a curve is a geometric line segment whose endpoints both lie on the curve

Diameter:
A diameter is a chord passing through the center. The length of a diameter is twice the radius. A diameter is the largest chord in a circle.

Secant Line:
A secant line of a curve is a line that (locally) intersects two points on the curve.

Tangent Line:
The tangent line through a point P on a circle is perpendicular to the diameter passing through P