(= Short Mount) 
 is a lens mount for use of mini-lenses with M12x0.5 thread (diameter = 12mm, 1 revolution = 0.5mm stroke. 

S-mount lenses are either used in special holders, or with adapters or in C-mount CS-mount cameras.

Like with C-mount, CS-mount and F-mount lenses diameter and thread pitch are fixed.
But different from these the back flange length (distance from the mechanical stop of the lens to the sensor) is NOT standardized.
This can lead to mechanical problems with filters mounted between the lens and sensor.