Single-Lens Translucent (SLT) is a Sony proprietary designation for Sony Alpha cameras which employ a pellicle mirror, electronic viewfinder, and phase-detection autofocus system.
Sony recently developed a new type of camera, the Single-Lens Translucent brand, that features a semi transparent fixed mirror. The mirror lets most of the light through to the sensor, and does not move for taking a picture, while redirecting some of the light up to a separate phase detect autofocus sensor. This allows for live view with fast phase detect autofocus, which is great for regular video and quick shots.
Cameras have a semi-transparent fixed mirror which diverts a portion of incoming light to a phase-detection autofocus sensor, while the remaining light strikes a digital image sensor. The image sensor feeds the electronic viewfinder, and also records still images and video on command. The utility of the SLT design is to allow full-time phase-detection autofocus during electronic viewfinder, live view, and video recording operation. With the advent of digital image sensors with integrated phase-detection, the SLT design is no longer required to accomplish this goal, as evidenced by cameras such as the Sony NEX-5R, Fujifilm X-100s, and Nikon 1.