Through the Lens, used when talking about either an autofocus or auto exposure system that works through the camera's lens. It's also (incorrectly) used to mean SLR, see "SLR" above.
(1.) A term used in digital electronics mainly to describe the ability of a device or circuit to be connected directly to the input or output of digital equipment. Such compatibility eliminates the need for interfacing circuitry. TTL signals are usually limited to two states, low and high, and are thus much more limited than analog signals. (2.) Thru-the-lens viewing or color measuring.
Acronym for Through The Lens, a system that reads the light passing through a lens that will expose film or strike an image sensor. A TTL system is useful when using accessories such as filters, extension tubes, and teleconverters, which reduce the amount of light reaching the film or sensor.