The repetitive sound a content cat makes is called purring. It is a low, rhythmic, vibrating vocalization produced by domestic cats and several other felids.
Cats can purr while being stroked, resting, eating, or nursing. Kittens begin purring very early in life, and the sound may help maintain contact between a mother and her young. A cat's purr is not always a simple sign of happiness: cats may also purr when frightened, injured, or trying to calm themselves.
Unlike meowing, which cats commonly use in communication with humans, purring is often produced during both inhalation and exhalation. Its fundamental frequency is commonly reported at about 25 hertz, creating the familiar buzzing sensation felt through the cat's body.
A common mix-up is treating purring as proof that a cat is healthy or relaxed. Context matters: body posture, ears, tail movement, and appetite provide better clues. The word “purr” can also describe similar sounds in some non-feline animals, but true purring is especially associated with cats.