rotten borough

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

• noun An election district having only a few voters but the same voting power as other more populous districts.

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

• noun archaic A borough that was represented in Parliament although it had very few voters.

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

• noun an English parliamentary constituency with few electors