In 19th century England, what was a "rotten borough"?

A clever trick indeed, a "rotten borough" was a mostly uninhabited area that could elect a member to parliament. The area most likely had been more populous at one time. The rub was that usually the most or whole area was owned by one person and since the other people that lived there could not vote against the owner, they basically had a free pass to parliament. The Reform Act of 1832 cleaned up a lot of these rotten boroughs.

https://en.wikipedia.org/wiki/Rotten_and_pocket_boroughs