Single rectangular strip of fabric (or other material) wrapping around the breasts to minimally cover the areolae and nipples.
Offers questionable breast support but can be stylish. Often creates skindentations.
Alternatively a garment can be considered a breast band if it at least has the appearance of being a single strip of fabric (even if it is not). The general point is that it has to "appear like a single band narrowly covering 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 generally not breast bands unless they appear to be a single strip of fabric.
All belt bras are technically breast bands but not all breast bands are belt bras.