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diameter
This term specifies a precise linear measurement that defines the maximum width of a circular object. It is a technical term used primarily in geometry, engineering, and physics to establish scale and proportion. While radius describes the distance from the center outward, diameter encompasses the full span. It is the primary metric used when specifying the size of cylindrical objects like pipes, bolts, or planetary bodies.
Countable when referring to the specific measurements of various objects (the diameters of several pipes). Uncountable when discussing the general geometric property of diameter in a theoretical sense.