adhesive bra
A bra or bikini top that has no shoulder straps or side bands, instead being held on only by adhesive. Also called a sticky bra, stick-on bra, or a NuBra, after the original brand name.
In real life, adhesive bras are designed to be worn with strapless, backless dresses. In fanart, they're more often worn by themselves or as part of a revealing outfit.