HD is short for High Definition. It is a term applied to many different screen displays.
 High Definition
Technically the term HD was coined to apply to the widescreen TV displays meeting the new digital TV video standards. There are two display values that can apply the term HD. HD can be either 1280 x 720 or 1920 x 1080 pixels. The latter is often referred to as Full HD, FHD. See Nomenclature to compare this designation to other screen resolutions. TV video can also be noted by the vertical resolution by stating 720P or 1080P as the screen resolution. The P in this case refers to Progressive scan. Both I (interlace scan) and P may be supported. P requires more bandwidth. A TV might not support 1080P at a frame rate above 24 frames per second.
FHD has a camera resolution of almost 2.1 Mpixel. This is often referred to as a 2K image based on a rounding of the horizontal resolution. Blu-Ray players support this format.
In eBook readers and tablets the term can specify a pixel size a bit larger that the minimum numbers noted above for example 1280 x 800 is often termed HD. An HD image could certainly be displayed at standard resolution on such a device. The term is also used to identify a high ppi which will be used on a 300 ppi screen (1440 x 1072 on a 6" device) or as a name change to represent a revised screen with higher ppi than an earlier version of the same model.
HD also defines an aspect ratio of 16:9 or in the some eBooks readers and computer screens 16:10. Any screen with an aspect ratio of 1.5 to 1 (3:2) or higher is termed a widescreen so HD is a widescreen but a widescreen may not be wide enough to be termed HD, however if the screen has enough width and height in pixels an HD image could be shown.
nHD (640 x 360) is the common denominator for FHD and HD. It is 1/3 width and 1/3 height of FHD (1/9 the screen size) while being 1/2 width and 1/2 height for HD (1/4 the screen size). The n stands for 1 ninth. You could display a 720p source on a 1080p screen by duplicating every other line. You could display a 1080p source on a 720p screen by removing every third line. Similarly you could also step down to SD (Standard definition) by repeating the same process of removing every third line, or display SD, 480i, on an HD screen by duplicating every other line.
While High Definition specifies the resolution in pixels it does not specify that depth of color to use. It could be only black and white, shades of gray, or color and still be HD. The usual grayscale is 16 levels while the typical color is a three dot pixel RGB with each color having 256 levels. See also HDR (High dynamic range).
Doubling both the horizontal and vertical of standard HD is called Quad HD with 2560 x 1440 resolution. Sometimes a + is added to widen the aspect ratio with more pixels or a prefix of W is added. The camera resolution would be almost 3.7 Mpixel.
As technology progresses a new form of HD is now available, called Ultra-High Definition or 4K, with both horizontal and vertical resolution of FHD doubled to 3840 x 2160. There is also HDR, High Dynamic Range, technology available on this resolution for increased contrast by allocating 10 bits per color in a 32bit word. This also increases the color range (gamut).
A variation called UHD+ 5K (5120 x 2880) is also available. It features 2880 as the height since this is the smallest number that evenly divides PAL (5) and NTSC (6), permitting exact translations between these two divergent systems.
In 2018 the newest format has been shown as 8K UHD sometimes called 8K Super UHD with a resolution of 7680 x 4320. It is expected to drive some of the newer compression schemes for video. Yes, Super Ulra-HD is a double superlative.