bandeau
A garment similar and related to the tube top, it is a narrow, band-shaped covering for the breasts. Its shape is used for swimsuit tops, undergarments and casual wear.
The design of the bandeau makes it easy for it to slip off the breasts, so a common addition is a halterneck to reduce the chance of slippage (example: post #700974). Some designs are front-tie, while others may feature O-rings.