Proportional representation is an electoral system in which the distribution of seats corresponds closely with the proportion of the total votes cast for each party. For example, if a party gained 40% of the total votes, a perfectly proportional system would allow them to gain 40% of the seats.
The Single Transferable Vote (STV) is a proportional voting system and enables the elector to list candidates in order of preference in a multi-member constituency. A candidate is elected once his votes reach the relevant quota and any excess votes over this quota are then transferred, according to the second preferences of the voters. This would not usually be the case in the First Past the Post system, used for Westminster elections. Different electoral systems achieve varying degrees of proportionality.