What is the definition of diameter?

The correct choice defines diameter as the thickness of something circular or cylindrical. The diameter is specifically the straight line segment that passes through the center of a circle or sphere, connecting two points on its boundary. It represents the largest distance across the circle, effectively measuring the width of that circular shape.

Understanding this concept is crucial, especially in contexts where measurements and calculations related to circular objects or structures are involved, such as construction, where proper measurement can impact safety and structural integrity. The other options do not accurately reflect the definition of diameter; they relate to other geometric properties, but they do not capture the essence of what a diameter is. The diameter is about the linear distance across a shape, not its thickness or volume.