A measure of the finest detail that can be seen, or resolved, in a reproduced image. Whilst it is influenced by the number of pixels in the display (e.g. high definition 1920 x 1080, broadcast SDTV 720 x 576 or 720 x 480) note that the pixel numbers do not define the resolution but merely the resolution of that part of the equipment chain. The quality of lenses, picture displays, edit systems and film scanners, etc., in fact any element in the program stream (from scene to screen), must be taken into account in assessing overall system resolution. Like a chain, it is only as good as its weakest link.
For decades television was only available in one analogue resolution in any one country. That improved with digital standard definition services (SDTV). Then HD at 1080 x 1920, with four-times the picture area, doubled the horizontal and vertical resolution. More recently 4K UHD doubles HD with 2160 x 3840 sized images. 8K UHD doubles those figures again.