gypsy cab

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

• noun A taxicab that is licensed only to respond to calls but often cruises the streets for passengers.

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

• noun colloquial, US An unlicensed taxi, especially in the North East of the United States.

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

• noun a taxicab that cruises for customers although it is licensed only to respond to calls