Piece of clothing or gear that minimally covers a bare crotch while leaving the pubic delta (front of the body right above the genitals) and the buttocks exposed.
Known forms include:
Rigid piece that hangs from the waist while snugging onto the crotch. Usually found on sci-fi or fantasy themed costumes.
Adhesive covers that are applied over the crotch area.
See also
C-String, a real-world strapless thong bikini bottom