A nursing bra is a specialized brassiere

Brassiere

A brassiere is an undergarment that covers, supports, and elevates the breasts. Since the late 19th century, it has replaced the corset as the most widely accepted method for supporting breasts....

 that provides additional support to women's milk-filled breast

Breast

The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants.Both men and women develop breasts from the same embryological tissues...

s and permits comfortable breastfeeding

Breastfeeding

Breastfeeding is the feeding of an infant or young child with breast milk directly from female human breasts rather than from a baby bottle or other container. Babies have a sucking reflex that enables them to suck and swallow milk. It is recommended that mothers breastfeed for six months or...

 without the need to remove the bra. This is accomplished by specially designed bra cups that include flaps which can be opened with one hand to expose the nipple

Nipple

In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...

. The flap is usually held closed with a simple clasp or hook.
Like its sister the maternity bra, a nursing bra is a practical bra designed with fuller cups, comfortable fabrics, and wider shoulder straps for increased comfort.