For Danbooru purposes, a flat chest is exactly what it says, i.e. no breasts whatsoever. For breasts larger than flat yet smaller than normal, use the small breasts tag.
Primarily for use with older-looking characters, though it may also be applied to loli characters. Not for use with child depictions.
The following tags are aliased to this tag: titty_buds, tsurupeta, flatchest, pettanko, flat_chested (learn more).
The following tags implicate this tag: flat_chest_grab (learn more).