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.
This tag may be used for traps where appropriate, because being a trap is generally not immediately obvious (hence the term) and they appear to be female.
The flat chest tag is mutually exclusive to the breasts tag, i.e. don't tag both unless another character in the picture meets the criteria for the breasts tag.
The following tags implicate this tag: flat_chest_grab.