A Camden Auction is an auction used in backgammon clubs whereby the price of a seat for participants in a tournament is auctioned up to the highest price that all are comfortable paying. The money paid for the seats by the players constitutes the prize money. The tournament itself is run as a single elimination (knockout tournament) with the prize money being awarded as a jackpot to the outright winner.
A more formal name for a Camden Auction is an "Entry Fee Auction Tournament". The colloquial name comes from the fact that these tournaments were first used in British backgammon at the Backgammon in Camden club, which is a part of the Backgammon in London organisation.
The format was devised by British tournament director Mike Main, who goes by the online handle MikeMadMonk, with the purpose of ensuring that as many people as possible get to play for money at a level that they can afford. With a sufficient pool of players it's possible to get two or three such entry fee auction tournaments going. The players who drop out of the Camden Auctions are often offered a place in a less costly tournament at, for example, the auctioned tournament's entry level or the price at which the first bidder dropped out.