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High-definition video is video of higher resolution than is standard. While there is no specific meaning for high-definition, generally any video image with more than 480 horizontal lines (North America) or 576 lines (Europe) is considered high-definition. 720 scan lines is generally the minimum even though many systems greatly exceed that. Images of standard resolution captured at rates faster than normal (60 frames/second North America, 50 fps Europe), by a high-speed camera may be considered high-definition in some contexts.

The first electronic scanning format, 405 lines, was the first "high definition" television system, since the mechanical systems it replaced had far fewer. From 1939, Europe and the US tried 605 and 441 lines until, in 1941, the FCC mandated 525 for the US. In wartime France, René Barthélemy tested higher resolutions, up to 1,042. In late 1949, official French transmissions finally began with 819. In 1984, however, this standard was abandoned for 625-line color on the TF1 network.

High definition video (prerecorded and broadcast) is defined threefold, by:

The number of lines in the vertical display resolution. High-definition television (HDTV) resolution is 1,080 or 720 lines. In contrast, regular digital television (DTV) is 480 lines (upon which NTSC is based, 480 visible scanlines out of 525) or 576 lines (upon which PAL/SECAM are based, 576 visible scanlines out of 625). However, since HD is broadcast digitally, its introduction sometimes coincides with the introduction of DTV. Additionally, current DVD quality is not high-definition, although the high-definition disc systems Blu-ray Disc and the defunct HD DVD are.

The scanning system: progressive scanning or interlaced scanning. Progressive scanning redraws an image frame (all of its lines) when refreshing each image, for example 720p/1080p. Interlaced scanning draws the image field every other line or "odd numbered" lines during the first image refresh operation, and then draws the remaining "even numbered" lines during a second refreshing, for example 1080i. Interlaced scanning yields greater image resolution if subject is not moving, but loses up to half of the resolution and suffers "combing" artifacts when subject is moving.

The number of frames or fields per second (Hz). In Europe more common (50 Hz) television broadcasting system and in USA (60 Hz). The 720p60 format is 1,280 × 720 pixels, progressive encoding with 60 frames per second (60 Hz). The 1080i50/1080i60 format is 1920 × 1080 pixels, interlaced encoding with 50/60 fields, (50/60 Hz) per second. Two interlaced fields formulate a single frame, because the two fields of one frame are temporally shifted. Frame pulldown and segmented frames are special techniques that allow transmitting full frames by means of interlaced video stream.

High-definition image sources include terrestrial broadcast, direct broadcast satellite, digital cable, high definition disc , digital cameras, internet downloads and the latest generation of video game consoles.

Most computers are capable of HD or higher resolutions over VGA, DVI, and/or HDMI.

The optical disc standard Blu-ray Disc can provide enough digital storage to store hours of HD video content. Digital Versatile Discs or DVDs (that hold 4.7 GB* for a Single layer or 8.5 GB* for a Double layer), look best on screens that are smaller than 36 inches (91 cm)[citation needed], so they are not always up to the challenge of today's high-definition (HD) sets. Storing and playing HD movies requires a disc that holds more information, like a Blu-ray Disc (which hold 25 GB* in single layer form and 50 GB* for double layer) or High Definition Digital Versatile Discs HD-DVDs which hold 15 GB* or 30 GB* in single and double layer. * = Gigabyte = 1,073,641,324 bytes, 1,048,576 killabytes, or 1024 megabytes.

Blu-ray Discs were jointly developed by 9 initial partners including Sony, Phillips (which developed CDs),and Pioneer (which developed its own Laser-disc previously with some success) among others. HD-DVD discs were primarily developed by Toshiba and NEC with some backing from Microsoft, Warner Bros., Hewlett Packard, and others. On February 19, 2008 Toshiba announced it was abandoning the format and would discontinue development, marketing and manufacturing of HD-DVD players and drives.

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