high-definition television (often shortened to high-definition and hd or high definition) describes a television system providing an image resolution of substantially higher resolution than the previous generation of technology. the term has been used since 1936, but in modern times refers to the generation following standard-definition television (sdtv), often abbreviated to hdtv or hd-tv. it is the current standard video format used in most broadcasts: terrestrial broadcast television, cable television, satellite television, blu-ray discs, and streaming video. the only difference is the picture quality of sd and hd. prior to this, respective sd versions of the channels transmits in 4:3 video with letterbox on it, making the image appears smaller.