The diameter of a circle is a line segment that passes through the center of the circle and has its endpoints on the circle. All the diameters of the same circle have the same length.
A chord is a line segment with both endpoints on the circle. The diameter is a special chord that passes through the center of the circle. The diameter would be the longest chord in the circle.
The radius of the circle is a line segment from the center of the circle to a point on the circle. The plural of radius is radii.
In the above diagram, O is the center of the circle and and are radii of the circle. The radii of a circle are all the same length. The radius is half the length of the diameter.
An arc is a part of a circle.
In the diagram above, the part of the circle from B to C forms an arc.
An arc can be measured in degrees.
In the circle above, arc BC is equal to the ∠BOC that is 45°.
In the above diagram, the line containing the points B and C is a tangent to the circle.
It touches the circle at point B and is perpendicular to the radius . Point B is called the point of tangency.
is perpendicular to i.e.
Congruent circles are circles that have the same radius, but different centers.
Two circles may intersect at two points or at one point. If they intersect at one point then they can either be externally tangent or internally tangent.
Two circles that do not intersect can either have a common external tangent or common internal tangent. In the common external tangent, the tangent does not cross between the two circles. In the common internal tangent, the tangent crosses between the two circles.This video introduces the features of circles including secant, secant line, chord, diameter, radius, tangent line, and intersecting circles.