The document defines key terms related to circles such as center, radius, and diameter. It states that the center of a circle is a fixed point, and the radius is the distance from the center to any point on the circle. The diameter is a line segment passing through the center whose endpoints lie on the circle. It provides the relationships that the diameter is twice the radius, and the circumference is approximately three times the diameter. Examples are given to illustrate calculating radius given diameter and circumference given diameter.