Hay is grass or legumes that has been cut, dried, stored, and used for animal feed, particularly for grazing animals like cattle, horses, goats and sheep. Small pets such as guinea pigs and rabbits also enjoy hay, though they only consume very small quantities. Pigs may be fed hay, but they do not digest hay very efficiently.

Hay is fed when or where there is not enough pasture or rangeland on which to graze an animal, when grazing is unavailable due to weather - such as during the winter, or when lush pasture by itself is too rich for easy digestion by the animal. It is also fed during times when an animal is unable to access pasture, such as when animals are kept in a stable or barn.