A low-pitched regular sound produced during alternating (pulmonic) egressive and ingressive airstream when the cat is content, hungry, stressed, in pain, gives birth or is dying; probably signals ”I do not pose a threat” or ”Keep on doing what you are doing”. Cat mothers and their kittens often use purring to communicate, as it is a quiet sound difficult for predators to detect. Some wild cats purr too, the most famous one probably being the Cheetah Caine (see video clip below). Sometimes purring is combined or merged with other sounds, like soft trills, squeaks or mews (see video clips below). 

The image to the right shows the phonetic characteristics (waveform, spectrogram and pitch (F0) contour) and gives a short audio example of a domestic cat purr.