backless panties
Panties that cover the pussy but that leave the ass exposed; sometimes worn with the intent of reducing pantylines.
If it's a swimsuit instead of panties, then use assless swimsuit instead.
If the panties have a back that covers most of the ass, but it has a single hole in it, use ass cutout instead.
See also
The following tags are aliased to this tag: assless_panties (learn more).
This tag implicates panties (learn more).