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.