Wikipedia
The T-mount is a standard lens mount for cameras and other optical assemblies. The usual T-mount is a screw mount using a male 42×0.75 (42 mm diameter, 0.75 mm thread pitch) metric thread on the lens with a flange focal distance of 55 mm and a mating female 42mm thread on a camera adapter or other optical component. This thread form is referred to as T-thread. (This should not be confused with the M42 lens mount which is also 42 mm diameter, but has a 1 mm thread pitch.)
The "T" is said to stand for Tamron or Taisei, a Japanese manufacturer that released in 1957 the first of a line of aftermarket camera lenses that fit 35 mm SLR cameras built by various manufacturers using their universal T-mount. On the first model, the mini T-mount used a M37×0.75 thread; Tamron’s canonical M42×0.75 T-thread didn’t appear on the market until about 1962. The company referred to it variously as a T-mount, T-thread, T-adapter, or a T-400, but not as a T-2, which is simply the name that Soligor used for its version of the T-adapter. The proprietary lens mount of each camera manufacturer was adapted to the T-mount thread with a simple adapter. Thus a retailer could stock a small number of expensive lenses that would fit a large number of camera brands using a selection of inexpensive adapters.
Some T-mount lenses accept a T-adapter that lacks its threaded inner ring, and simply slides onto the T-mount lens. It is secured only by 3 set screws that match a channel on the lens barrel, such that the barrel is not deformed by the set screws and the adapter can therefore be removed with no damage.
As a common mechanical interface, the T-mount allows components of various manufacturers to be interchanged and assembled. The T-mount is a solely mechanical specification. Electrical or mechanical connections (such as for autofocus) are generally not provided, although Sigma’s YS Mount featured an automatic diaphragm coupling.
Besides cameras, T-mounts are used in optical breadboard prototyping components as well as telescope and microscope attachments. The T-mount is also a standard way to mount a camera to a microscope to photograph pathological specimen slides or to a 1.25″ telescope eyepiece.