The document defines key terms related to circles, including circumference, which is the distance around the circle; center (or centre), which is the midpoint of a circle; radius, which is the line drawn from the center to the circumference or from one point on the circumference through the center to another point; diameter, which cuts the circle in half and is a special type of chord; chord, which connects two points on the circumference; and arc, which is a part of the circumference.