A ruminant is a mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

 of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud
Cud is a portion of food that returns from a ruminant's stomach in the mouth to be chewed for the second time. More accurately, it is a bolus of semi-degraded food regurgitated from the reticulorumen of a ruminant. Cud is produced during the physical digestive process of rumination, or "chewing the...

, and chewing it again. The process of rechewing the cud to further break down plant matter and stimulate digestion is called "ruminating".

There are about 150 species of ruminants which include both domestic and wild species.