NOUN
rotten boroughs (plural noun)
a borough that was able to elect an MP despite having very few voters, the choice of MP typically being in the hands of one person or family.
Before the great reform act of 1832 a rotten borough was one with very few or no voters. The borough of Dunwich had actually fallen into the sea many years before yet still sent an MP. Not to be confused with a pocket borough in which the MP was chosen by the local landowner.