monokini; one-piece swimsuit with exposed back
A one-piece swimsuit that leaves the back largely bare, often with high-cut legs. The term is a blend of 'mono-' and 'bikini'.
She was wearing a monokini at the beach.
bikini briefs
Refers specifically to the bottom piece of a bikini; less common than the swimsuit sense.
These bikini briefs are worn under a swimsuit.