From Edmund Optics website:

M12 or micro-video imaging lenses are typically board lenses that are small and feature an S-Mount type camera or lens mount. The S-Mount is a standard lens mount used in machine vision and M12 refers to the metric thread type with a nominal diameter of 12mm, a pitch of 0.5mm (M12 x 0.5), and without a standardized flange distance (see The Anatomy of a Lens) as many other mount type have including the C-Mount and TFL-Mount for example. These lenses are often ideal for space- and weight-constrained applications including OEM implementation, CCTV, and other board-level cameras. Additionally, M12 lenses are low cost alternatives to standard-sized fixed focal length lenses. For more information on imaging lenses and their differences, read Types of Machine Vision Lenses.

From the Edmund Optics website, some of their M12 lenses cover sensors up to 1/2", while others have a maximum sensor size of 1/1.8". The M12 standard is a mount size and does not specify the sensor size, so M12 lenses could be made for other sensor sizes as well.