A sum of money paid by candidates in an election. The deposit is forfeited in the event of a candidate not gaining (polling) a certain percentage of the total votes. Deposits are supposed to deter candidates who are not serious from standing for parliamentary election. The deposit is currently £500 and is forfeited if a candidate polls less than 5% of the vote.
Further information on this subject can be found from the following link.
Background information - An Introduction to Parliament: Elections