Final answer:
The diameter of a needle is expressed in terms of its gauge, which is an inverse measurement where a larger gauge indicates a smaller diameter.
Step-by-step explanation:
The diameter of a needle is expressed in terms of its gauge. The gauge number is inversely related to the diameter, meaning the larger the gauge number, the smaller the diameter of the needle. Medical professionals use the gauge to determine the appropriate needle size for various procedures, ensuring that the fluid flow rate and patient comfort are properly managed.