QHD, Quad HD or 2K
QHD stands for Quad HD, or four times the definition of standard HD. The pixel measurement for QHD is 2,560 x 1,440 which is where the 2K name comes from. Definitions are often shortened to the smaller number of the pixel measurement, so HD will sometimes be called 720p, Full HD is 1,080p and so on. With QHD though, the 2K name comes from the fact that the bigger of the pixel measurements is over 2,000 pixels. This can be a bit confusing. A 5.5-inch QHD display has a pixel density of 538 ppi.
Ultra HD or 4K
You can probably see where this is going. Like 2K, the 4K name comes from the larger of the two pixel measurements, which are, technically speaking, 4,096 pixels in 4K and only 3,840 pixels in Ultra HD. So while these two definitions are often said in the same breath they are actually a little bit different. Ultra HD is 3,860 x 2,160 pixels and 4K is 4,096 x 2,160. Of course, both definitions frequently get shortened to 2,160p. You can probably see that calling QHD a 2K display is technically incorrect, as it should really be called 2.5K