The gauge of a hypodermic needle refers to its:

The gauge of a hypodermic needle specifically refers to its diameter. In the context of needles, gauge is a standardized measurement that indicates how thick or thin the needle is. A larger gauge number corresponds to a smaller diameter, while a smaller gauge number indicates a larger diameter. This relationship is critical in various medical scenarios; for example, a thicker needle (lower gauge) is often used for procedures that involve thicker fluids, while a thinner needle (higher gauge) is suitable for less viscous substances.

Length, use, and radius do not accurately define gauge in this context. Length pertains to how long the needle is, while use pertains to the applications for which the needle is intended. Radius is a specific measurement of the needle's thickness that is not typically referred to in practice; gauge serves as a more straightforward and widely understood indicator of the needle's size.