Source : Webster's Revised Unabridged Dictionary (1913)

Purr \Purr\, v. i. & t.
   To murmur as a cat. See {Pur}.

Purr \Purr\, n.
   The low murmuring sound made by a cat; pur. See {Pur}.

Source : WordNet®

purr
     n : a low vibrating sound typical of a contented cat
     v 1: make a soft swishing sound; "the motor whirred"; "the car
          engine purred" [syn: {whizz}, {whiz}, {whirr}, {whir}, {birr}]
     2: indicate pleasure by purring; characteristic of cats [syn: {make
        vibrant sounds}]