breast band
Single rectangular strip of fabric (or other material) wrapping around the breasts to minimally cover the areolae and nipples.
Differs from a tube top and a bandeau in that a breast band does not offer full coverage of the breasts and must reveal underboob and sideboob to be considered a breast band. The fabric from a breast band should comparatively only barely cover the areolae and nipples while being a perfectly rectangular strip.
Differs from a strapless bra and a strapless bikini in that the fabric does not form cups for the individual breasts and remains a single continuous strip of fabric that tightly secures the breasts.
Sarashis are not breast bands unless it is a single strip of fabric (as opposed to multiple overlaid strips as is usual for a sarashi).
Belt bras are not breast bands.