Species of cattle Bos grunniens, family Bovidae, which lives in wild herds at high altitudes in Tibet. It stands about 2 m/6 ft at the shoulder and has long shaggy hair on the underparts. It has large, upward-curving horns and humped shoulders. It is in danger of becoming extinct.

In the wild, the yak is brown or black, but the domesticated variety, which is half the size of the wild form, may be white. It is used for milk, meat, leather, and as a beast of burden. The yak is protected from extremes of cold by its thick coat and by the heat produced from the fermentation in progress in its stomach.