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HD
Can mean a particular number of dots, ie. 1920 x 1080. But 1920 x 1080 isn't really HD if the image is 72″ and 1m away.
Converesly System A TV, 405 lines, was about equivalent to about 320 x 377 and was HD in 1935, because 6" to 9" 5:4 screens using round tubes were common and it replaces 22 line to 180 line mechanical systems.
The USA 525 lines (VGA based on it so 640 x 480) was designed to be roughly equivalent to 16mm film. The 35mm is about x4 resolution in 1940s but was used (is still occasionally) with much larger screens and modern film stock needs scanned at about 8K to give post processing UHD (4K). Some cheaper BD is simply the 2K HD scan used as master for SD DVD instead of re-scanning at 4K.
HD has become slightly meaningless and HD Ready was always a marketing scam.
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