AXIS have a pretty good write up on resolution - it's not a legal standard but an adaption of older analog standards. http://www.axis.com/files/feature_articles/ar_perfect_pixel_count_47927_en_1212_lo.pdf
'Pixel density' is a good measure of usable resolution but you still have to consider the usability of the image, eg: are details clear & sharp etc.
To measure: put a ruler in the picture at the desired distance, take a snapshot, count the pixels and multiply to find pixels per foot / meter..