HDTV stands for High-Definition Television and as its name suggests, it specifies a higher resolution to the viewable TV image than the existing widely-used "standard" TV formats, such as NTSC and PAL.
Term HDTV is very often confused with the term DTV which simply stands for digital TV, but doesn't actually define higher resolution than the existing resolutions of PAL and NTSC. This "normal" DTV standard is widely used in Europe with normal PAL resolution of 704x576. European-wide standardization of HDTV specs is still largely under discussion and it is likely that Europe will switch to digital without actually changing the resolution to a higher one.
The specified HDTV resolutions that are used in United States and most notably in Japan, are called 1080i, 720p and 720i. HDTV's native aspect ratio is 16:9 -- same as with anamoprhic DVD-Video discs, but in HDTV the resolution is "really" in 16:9 and not achieved by strecthing the pixels to be wider, like in anamorphic DVD-Video discs.
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