The Collaborative International Dictionary of English v.0.48:

Pur \Pur\, v. i. [imp. & p. p. Purred; p. pr. & vb. n.
   Purring.] [Of imitative origin; cf. Prov. G. purren.]
   To utter a low, murmuring, continued sound, as a cat does
   when pleased. [Written also purr.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pur \Pur\, n.
   The low, murmuring sound made by a cat to express contentment
   or pleasure. [Written also purr.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Purr \Purr\, n.
   The low murmuring sound made by a cat; pur. See Pur.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Purr \Purr\, v. i. & t.
   To murmur as a cat. See Pur.
   [1913 Webster]

WordNet (r) 3.0 (2006):

purr
    n 1: 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, purr]
    2: indicate pleasure by purring; characteristic of cats [syn:
       purr, make vibrant sounds]