The dimension of something that fits into something else, like the bottom diameter of a chimney that fits into a gallery, or the bottom diameter of a burner base that fits into a collar
A fitter is a person whose job is to put together, adjust, or install machinery or equipment. George was a fitter at the shipyard. someone who puts together or repairs machines or equipment
A skilled mechanical engineer who ‘fitted’ parts together These parts would be relatively small and requiring accuracy, such as mechanical components of a (steam, or internal combustion) engine rather than large sheet metal parts
The "lip" part of a glass shade or globe that attaches to the shade-holder part of the fixture Common fitter sizes (diameters) are 2-1/4" 3-1/4", 4" and 6" Typically, the fitter measures slightly less than these dimensions