A purr is a tonal fluttering sound made by some species of felids, including both larger, outdoor cats and the domestic cat (Felis catus), as well as two species of genets. It varies in loudness and tone among species and in the same animal.

Although true purring is exclusive to felids and viverrids, other animals such as raccoons produce vocalizations that sound similar to true purring. Animals that produce purr-like sounds include mongooses, bears, kangaroos, wallabies, wallaroos, badgers, foxes, hyenas, rabbits, squirrels, guinea pigs, tapirs, ring-tailed lemurs and gorillas while eating. Animals purr for a variety of reasons, including to express happiness or fear, and as a defense mechanism. It has also been shown that cats purr to manage pain and soothe themselves. Purring is a soft buzzing sound, similar to a rolled 'r' with a fundamental frequency of around 25 Hz. This sound occurs with noticeable vibrations on the surface of the body, varies in a rhythmic pattern during breathing and occurs continuously during inhalation and exhalation. The intensity and length of the purr can also vary depending on the level of arousal of the animal.