The Sigma SA mount is a design of lens mount

Lens mount

A lens mount is an interface — mechanical and often also electrical — between a photographic camera body and a lens. It is confined to cameras where the body allows interchangeable lenses, most usually the single lens reflex type or any movie camera of 16 mm or higher gauge...

 designed by the Sigma Corporation

Sigma Corporation

is a Japanese company founded in 1961, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan...

 of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 for use on their single-lens reflex camera designs. The SA mount uses a bayonet mount

Bayonet mount

A bayonet mount or bayonet connector is a fastening mechanism consisting of a male side with one or more pins, and a female receptor with matching L slots and spring to keep the two parts locked together....

 which is physically similar to the Pentax K mount

Pentax K mount

The Pentax K mount, sometimes referred to as the "PK mount", is a lens mount standard for mounting interchangeable photographic lenses to 35 mm single-lens reflex cameras. It was created by Pentax in 1975, and has been used by all Pentax 35 mm and digital SLRs since...

 but uses a flange focal distance

Flange focal distance

For an interchangeable lens camera, the flange focal distance of a lens mount system is the distance from the mounting flange to the...

 of 44 mm, identical to that of the Canon EF lens mount

Canon EF lens mount

Introduced in 1987, the EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens...

. The mount uses electrical communication between body and lens, like the EF mount, and in fact uses the same signalling lines and protocol as the EF mount, despite the physical incompatibility. Sigma has long produced EF-mount lenses for Canon cameras, and thus had the ability to use this protocol.
All Sigma SLRs and DSLRs can use manual focus lenses too, which have no electrical communication between body and lens. Nikon mount and M42 mount lenses can be fitted and used via easily available lensmount adapters. Pentax K mount (PK) lenses can be fitted straight onto the SA mount, after the protruding AA lever and AA protector outcrop are removed, though the fit is slightly loose as the SA mount has a slightly bigger bore than the PK mount.
Some Sigma DSLR users have become adept at converting high quality manual focus lenses to fit the SA mount. One such user, in the UK, has successfully converted Nikon, Contax Carl Zeiss, Minolta MD and Canon FL and FD lenses to fit the SA mount.
Others have even produced replacement mounting plates for Sigma DSLRs, which convert them to either Nikon or Leica R mount.

Camera Bodies

Body	Release Date	Sensor
Film (36 x 24 mm)
SA-300	1992	n/a
SA-300 N	1994	n/a
SA-5	1997	n/a
SA-7	2001	n/a
SA-9	2001	n/a
Digital (20.7 x 13.8 mm)
SD9 Sigma SD9 The Sigma SD9 is a digital SLR camera produced by the Sigma Corporation of Japan. The camera was launched at the Photo Marketing Association Annual Show on February 18, 2002. It was Sigma's first digital camera, and was the first production camera to use the unique Foveon X3 image sensor, which...	Oct. 2002	Foveon X3 Foveon X3 sensor The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics.... , 2268 x 1512 pixels
SD10 Sigma SD10 The Sigma SD10 is a digital SLR camera produced by the Sigma Corporation of Japan. It was announced on October 27, 2003 and is an evolution of the previous SD9 model, addressing many of the shortcomings of that camera...	Dec. 2003	Foveon X3 Foveon X3 sensor The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics....  with microlenses, 2268 x 1512 pixels
SD14 Sigma SD14 The Sigma SD14 is a digital single-lens reflex camera produced by the Sigma Corporation of Japan. It is fitted with a Sigma SA mount which takes Sigma SA lenses....	Mar. 2007	Foveon X3 Foveon X3 sensor The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics.... , 2640 x 1760 pixels
SD15 Sigma SD15 The Sigma SD15 is an updated version of Sigma SD14 DSLR produced by the Sigma Corporation of Japan and featuring the improved TRUE II image processing engine, but with the same image sensor as its predecessor. As such, the SD15 features the 14.7MP Foveon X3 sensor...	Jun. 2010	Foveon X3 Foveon X3 sensor The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics.... , 2640 x 1760 pixels
Digital (24 x 16 mm)
SD1 Sigma SD1 The Sigma SD1 is a digital SLR camera produced by the Sigma Corporation of Japan. The camera uses a Foveon X3 sensor, which comprises 3 layers of 4800 x 3200 pixels , giving much higher spatial resolution than the equivalent Bayer array...		Foveon X3 Foveon X3 sensor The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics.... , 4800 x 3200 pixels

Lenses

Most of the lenses manufactured by Sigma

Sigma Corporation

is a Japanese company founded in 1961, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan...

 are available for the SA mount. A partial listing can be found here.