Actually, I've seen 2560x1440 called "2.5K" (at least Blackmagic calls it that in their Cinema Camera which shoots 2432 x 1366), since 2K has a well established

Actually, I've seen 2560x1440 called "2.5K" (at least Blackmagic calls it that in their Cinema Camera which shoots 2432 x 1366), since 2K has a well established definition of 2048x1080, or some crop of that to meet a desired aspect ratio. In film, 4K is defined as 4096x2160, which is exactly 4 times the number of pixels as 2K.