"Background" as of wiki definition is used for "full background scenes". Proposal is to use "full_background" for such scenes, so as to make it distinct form the other types of backgrounds as noted in the wiki definition. After retagging, background should be aliased to "invalid_tag" since practically all images would theoretically have a background of some sort.
(I'd suggest an alias, but there's quite a few images which are mistagged background as per wiki definition.)
Updated by SnowWolf