"M12 lenses are easily recognized by their small size and the thread on the lens barrel. The M12 refers to the metric thread diameter (12 mm), which is much smaller than C/CS-mount lenses. These are often used in compact and modular camera models."
— Axis Communications Academy, Network Video Fundamentals, 2022
"M12-mount lenses are also called 'board lenses' and are screwed directly into the camera’s sensor module."