A bra without cups; also called a open-cup bra. 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.
Not to be confused with nursing bras, which cover the breast/nipple but still allow one to conveniently access the nipple through a slit.