What classifies a mammal as a ruminant?

A ruminant is an herbivorous mammal that has two stomachs. After chewing food, usually grass, they swallow it and it ferments in the first stomach. Then they regurgitate the food, which is now called cud, and chew it a second time, before swallowing it again, at which point the cud moves to the second stomach, where digestion continues. Most ruminants belong to the order Ruminantia, but not all of them do.