A firm-fitting undergarment, usually worn as lingerie, designed to push up the breasts. A bustier resembles a bra, but longer, extending beneath the breasts to cover part of the waist and stomach. The length may vary, but a bustier usually ends above the navel.
The difference between a bustier and a corset is that a corset has laces or buttons in front, is usually longer and made of stronger material, usually doesn't cup the breasts, and is usually is tighter around the waist. A bustier is usually shorter (above the navel), usually cups the breasts, and is designed mainly to hold up the breasts, not to hold in the waist.
In addition, a bustier should not be confused with an underbust, which denotes attire worn below -and not on- the breasts.