The term used for the relationship that exists between two mating parts is "fit". In engineering, the fit refers to the range of tightness or looseness between two mating parts, such as the fit between a shaft and a bearing or the fit between two interlocking mechanical components. The fit can be categorized as clearance fit, interference fit, or transition fit, depending on the amount of overlap or gap between the parts. Achieving the right fit is critical for the proper functioning of mechanical assemblies and to prevent issues such as excessive wear, vibration, or failure.