The document defines key terms related to circles: - A circle has a center, radius, diameter, and circumference. A chord and secant intersect a circle at two points, while a tangent intersects at one point. - Angles can be central, inscribed, interior inscribed, exterior inscribed, depending on whether their vertex is at the center or on the circle. - Other terms defined are arc, intercepted arc, and sector, which refer to portions of the circle.