A bikini (top) that lacks any shoulder straps or halternecks, instead relying on the tightness of the fabric to keep it in place, similarly to a tubetop. Most bandeaus (post #3514676, post #3527890, post #3508662) and eyepatch bikinis (post #3457055, post #3295380) are examples of strapless bikinis.
This tag implicates bikini and strapless (learn more).