bok choy, (Brassica rapa), member of the mustard family (Brassicaceae) that is a variety (chinensis) of Brassica rapa.
Bok choy belongs to a family of plants that includes other vegetables popular in Asian cookery such as mustard greens and Chinese leaves (also known as Chinese cabbage and Napa cabbage), as well as broccoli, Brussels sprouts, and kale. Its name has as many different spellings—pak choi and bak choi are two of the most common—as it boasts different varieties. Some have lighter green leaves and thicker stems; with others the leaves are heavily veined with white, and the sizes can range from just over 1 inch (2.5 cm) up to about 8 inches (20 cm). However, bok choy generally has a distinctive appearance: green, ruffled leaves contrast sharply with the smooth, juicy white stems that give the vegetable its name (bok is the Cantonese for “white”).
The attractive white-green contrast means that the vegetable is often cooked whole, while slightly larger ones are halved or quartered lengthwise. The stems are juicy and crunchy, whereas the thin leaves wilt quickly when cooked. Bok choy can be boiled, steamed, stir-fried, and used to fill dumplings. The flavour of all bok choys are subtle compared to many other members of Brassicaceae—mildly sweet and sometimes with a faintly bitter undertone. Bok choy also tends to absorb the flavours of foods cooked with it, making it a favourite addition to stir-fried dishes and soups.