Any of several ancient styles of garment designed to cover the genitals. Usually made from a folded and tied length of cloth, sometimes held in place by a belt. If it has a dangling length of cloth hanging in front and/or or behind, that part is called, for Danbooru's purposes, a pelvic curtain.
Both loincloth and fundoshi nowadays are used as festival clothing (usually with happi, or alone) , as a swimsuit (when worn at the beach) and occasionally, as underwear (usually worm under kimono, or Japanese armor etc). Make sure to use the appropriate tag when tagging them.