How is "definition" (SD, HD, UHD) actually defined?
The problem is that definitions such as HD, UHD and 4k etc. are partly hype. Marketers for AV equipment like to be rather elastic with the truth, hence the rather rubbery definitions. They're not standards such as ATSC or DVB, or graphic display resolutions such as VGA, WXGA etc., they're really just buzzwords.

There are other complications - like the different broadcast standards for SD — so that SD PAL is 576 visible lines and SD NTSC is 480 lines — or non-square pixels in digital video meaning that even a 16:9 HD 1080p image can be either 1440x1080 or 1920x1080*. So it's pretty understandable that you're confused.

For what it's worth, the general consensus is that:


SD refers to 576i / 480i (with the unusual EDTV variant being 576p and 480p). Display resolution is 768 × 576 PAL or 720 × 534 NTSC, often the actual pixel resolution is reduced, as in the case of DV: 720 × 576 PAL or 720 × 480 NTSC, with non-square pixels
HD refers to 720p and 1080i / 1080p. The display resolution is 1280 × 720 or 1920 × 1080, but sometimes the horizontal pixel resolution is reduced, e.g. HDV which is 1440 × 1080 with non-square pixels. You could also say that resolutions higher than this are also High Definition. So in one sense it's anything that isn't standard definition. Note that there is no 720i.
Full HD or FHD means the 1080 flavour of HD (either interlaced or progressive)
2K describes the Digital Cinema Initiative resolution of 2048 × 1080, this resolution is based on the historical resolution of scanned super 35mm film. As with 4K it is a little slippery, and can sometimes be used to describe FHD. As the wikipedia article states it describes content "having horizontal resolution on the order of 2,000 pixels". So in other words: near enough is good enough.
QHD or Quad HD - 2560 × 1440 is a whacky one, it's twice the resolution of 720p. The "quad" refers to there being four times the pixels of a 720p frame. Not to be confused with QFHD - see below.
UHD refers to 3840 × 2160, and like HD also sometimes to higher resolutions like 4K and UHD 8K
QFHD is 3840 x 2160 "Quad Full HD", the quad referring to the fact that it has four times the pixels of a FHD frame, which sounds better than saying it has twice the horizontal and vertical resolution. QFHD = UHD
UHD 4K - 3840 × 2160, is double the HD 1080 frame size, and is also sometimes called 4K, even though the horizontal resolution is less than 4000 pixels. UHD 4K = UHD
UHD 8K or FUHD is 7680 × 4320. I'm guessing that as the FUHD sets hit the shops they'll be called "8K" by the salespeople.
4K usually refers to Digital Cinema Initiatives 4K which is 4096 × 2160 but the term is sometimes used to refer to UHD 4K
Similarly 8K should properly refer to 8192 × 4320 but is also applied to FUHD


Note that there are cameras such as the Red cameras that shoot in-between resolutions such as 3K, 5K and 6K. But that's just for production, the finished media will usually be cropped / scaled to a more standard resolution for display or broadcast.

TL;DR If you're trying to define anything in technical terms and want to be unambiguous just give the pixel dimensions; terms like HD, UHD etc are really FUD. They're more for marketing than anything else.

Edit: Here's a handy chart:



pdf version available here