What is the definition of diameter?

The definition of diameter is a line passing through the center of a circle or sphere. This definition highlights that the diameter not only connects two points on the boundary of the circle but also does so by going directly through the center point. It effectively divides the circle into two equal halves, which is a crucial aspect of its geometric characteristics. Understanding the diameter is essential in various mathematical applications, including calculating the area and circumference of circles, as it is used in those formulas.

Although the other provided options may contain related concepts—such as the line connecting two points on the circle or the distance around the circle—they do not accurately convey the complete definition of a diameter itself. The area of a circle, mentioned, is an entirely different property, calculated based on the radius or diameter, but does not define what a diameter is.