Minimax simply means "minimizing the maximum error". The lowest value of largest error is acheived there. For example, 1/4-comma meantone is the "minimax" meantone with respect to the 5-limit, as all 5-limit intervals are approximated with a maximum error of 1/4-comma, and no other meantone can improve on that (see the "historical meantones" chart at the bottom of Joe's meantone definition page to see why).