I don't think 2K really has any relevance in monitor marketing - even when it is mentioned, it's mentioned in conjunction with something else.
There should only really be
HD/720p / 921,600 pixels
FHD/1080p / 2,073,600 pixels
WQHD / 1440p / 3,686,400
4K / 3840x2160 / 8,294,400 pixels
.....and wide-screen variants.
In that format, pixel count ~doubles per 'step' up, and is fairly clear/straightforward.
2K is marketing blurb for TVs (where pixel count increased <10%), that has been misappropriated into the monitor space, IMO.
2K displays are those whose length falls in the 2,000-pixels range. The Digital Cinema Initiatives (DCI), a group of motion picture studios that creates standards for digital cinema, defines Standard DCI 2K resolution as 2048 x 1080.