"2K resolution" is a generic categorical term which can include any number of specific resolutions. Some examples include:
2048 × 1556, the resolution used for 2K scans of 35 mm film[2]
2048 × 1080, the resolution of the DCI 2K distribution standard
1998 × 1080, the resolution used for 1.85∶1 ratio 2K content in the DCI standard