What is defined as the tightest possible fit between two mating parts?

The term that refers to the tightest possible fit between two mating parts is actually "Interference." This fit occurs when the dimensions of the parts are such that they cannot be assembled without some amount of force, resulting in a press fit. Interference fits are typically used when high precision and strength in the joint are required, such as in gears or bearings where components must be tightly secured to prevent relative motion.

Clearance fit, on the other hand, allows for a gap between the parts, ensuring easy assembly and movement. Allowance is a concept that relates to the intentional deviation in size used to create specific types of fits, but it does not define the fit itself. Nominal size refers to the designated size of a part, without any reference to the specific fit type, thus not representing the tightest fit between parts.

Interference is fundamentally about achieving a secure connection that prevents relative movement, making it the correct term for describing the tightest possible fit.