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 tube top. Most bandeaus (post #3514676, post #3527890, post #4475028) and eyepatch bikinis (post #3457055, post #3295380) are examples of strapless bikinis.
This tag implicates bikini and strapless (learn more).