High-definition television (HDTV) provides a resolution that is substantially higher than that of standard-definition television.

HDTV may be transmitted in various varieties:

The letter "p" here stands for progressive scan while "i" indicates interlaced.

When transmitted at two megapixels per frame, HDTV provides about five times as many pixels as SD (standard-definition television).

The term high definition once described a series of television systems originating from the late 1930s; however, these systems were only high definition when compared to earlier systems that were based on mechanical systems with as few as 30 lines of resolution.

The British high-definition TV service started trials in August 1936 and a regular service on 2 November 1936 using both the (mechanical) Baird 240 line and (electronic) Marconi-EMI 405 line (377i) systems. The Baird system was discontinued in February 1937. In 1938 France followed with their own 441-line system, variants of which were also used by a number of other countries. The US NTSC system joined in 1941. In 1949 France introduced an even higher-resolution standard at 819 lines (768i), a system that would be high definition even by today's standards, but it was monochrome only. All of these systems used interlacing and a 4:3 aspect ratio except the 240-line system which was progressive (actually described at the time by the technically correct term "sequential") and the 405-line system which started as 5:4 and later changed to 4:3. The 405-line system adopted the (at that time) revolutionary idea of interlaced scanning to overcome the flicker problem of the 240-line with its 25 Hz frame rate. The 240-line system could have doubled its frame rate but this would have meant that the transmitted signal would have doubled in bandwidth, an unacceptable option.