A bra without a cup; also called a open-cup bra. It has little to no useful purpose beyond eroticism. Designs vary widely, but in general the design will encircle the breast, and will push the breasts upwards and outwards. May or may not cover the nipples and/or areolae.
For bras that are only an underwire "shelf" that holds up the breast see the shelf bra tag.
This tag implicates bra (learn more).