A full-frame digital SLR is a digital single-lens reflex camera (DSLR) fitted with an image sensor that is the same size as a 35 mm (36×24 mm) film frame. This is in contrast to cameras with smaller sensors, typically of a size equivalent to APS-C-size film, much smaller than a full 35 mm frame. Currently, the majority of digital cameras, both compact and SLR models, use a smaller-than-35 mm frame, as it is easier and cheaper to manufacture imaging sensors at a smaller size. Historically, the earliest digital SLR models, such as the Nikon NASA F4 or Kodak DCS 100, also used a smaller sensor.
Read more about Full-frame Digital SLR: Use of 35 Mm Film-camera Lenses, Advantages and Disadvantages of Full-frame Digital SLRs, Prototype Full-frame Digital SLRs