A circle is defined as the set of all points equidistant from a given center point. The radius is the distance from the center to the edge, the diameter starts at one side, goes through the center, and ends on the other side, and the circumference is the distance around the edge. Key lines related to a circle include chords, diameters, tangents, and arcs. Circles can also be divided into sectors such as quadrants and semicircles.