The lens mount used by Sony’s older Alpha SLT cameras. These are still on sale (2020) but are no longer being actively developed.
Because these cameras have a mirror in the body, even though it’s a non-moving one, the rear of the lens is further from the sensor, so Alpha mount lenses are physically different to Sony’s E-mount lens range. You can use Alpha lenses on an E-mount camera with a lens adaptor, but not the other way round.