purr

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The soft vibrant sound made by a cat.
• noun A sound similar to that made by a cat.
• intransitive verb To make or utter a soft vibrant sound.
• intransitive verb To express by a soft vibrant sound.

from The Century Dictionary.

• noun The sound made by a cat in purring.
• To utter a low murmuring sound expressive of satisfaction or pleasure, as a cat.
• To express or signify by purring.
• noun See pur.
• noun A sandpiper, Tringa alpina, commonly called dunlin.
• noun A bivalve of the family Veneridæ, Tapes decussata.

from the GNU version of the Collaborative International Dictionary of English.

• noun The low murmuring sound made by a cat; pur. See pur.
• verb To murmur as a cat. See pur.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb intransitive Of a cat, to make a vibrating sound in its throat when contented.
• verb transitive To say (something) in a throaty, seductive manner.
• verb intransitive To make a vibrating throaty sound, as from pleasure.
• verb intransitive, of an engine To make a low and consistent rumbling sound.
• noun The vibrating sound made by a cat in its throat when contented.
• noun A throaty, seductive sound of pleasure made by a person.
• noun The low consistent rumble made by an engine at slow speed

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb indicate pleasure by purring; characteristic of cats
• verb make a soft swishing sound
• noun a low vibrating sound typical of a contented cat