An electoral district (also known as a constituency, riding, ward, division, electoral area or electorate) is a distinct territorial subdivision for holding a separate election for one or more seats in a legislative body. However not all political systems use separate districts to conduct elections; Israel and the Netherlands, for instance, conduct parliamentary elections as a single, nationwide entity. In contrast, the United Kingdom, France and many other nations elect each member of the legislature from their own individual district. Generally, only voters who reside within the geographical bounds of an electoral district are permitted to vote in an election held there.
The names for electoral districts vary across countries and, occasionally, for the office being elected. The term constituency is commonly used to refer to an electoral district, however this term can also refer to the body of eligible voters within the represented area. Similarly, in Australia and New Zealand, electoral districts are called electorates, however elsewhere the term electorate generally refers specifically to the body of voters. In Canada, districts are colloquially called ridings (stemming from an earlier British geographical subdivision). Local electoral districts are sometimes called wards, a term which also designates administrative subsidivisions of a municipality. In Irish local government voting districts are called electoral areas.
District magnitude is the number of representatives elected from a given district to the same legislative body. A single-member district has one representative, while a multi-member district has more than one. Voting systems that seek proportional representation (such as the single transferable vote) inherently require multi-member districts, and the larger the district magnitude the more proportional a system will tend to be (and the greater the number of distinct parties or choices that can be represented.)
Under proportional representation systems, district magnitude is an important determinant of the makeup of the elected body. With a larger number of winners candidates are able to represent proportionately smaller minorities; a 10% minority in a given district may secure no seats in a 5 member election but would be guaranteed a seat in a 9 member one because they fulfill a Droop quota. The geographic distribution of minorities also affects their representation - an unpopular nationwide minority can still secure a seat if they are concentrated in a particular district. District magnitude can sometimes vary within the same system during an election. In the Republic of Ireland, for instance, national elections to Dáil Éireann are held using a combination of 3, 4, and 5 member districts.
Apportionment is the process of allocating a number of representatives to different regions, such as states or provinces. Apportionment changes are often accompanied by redistricting, the redrawing of electoral district boundaries to accommodate the new number of representatives. This redrawing is necessary under single-member district systems, as each new representative requires their own district. Multi-member systems, however, vary depending on other rules. Ireland, for example, redraws its electoral districts every election while Belgium uses its existing state boundaries for electoral districts and instead increases the number of representatives alloted to each.
Apportionment is generally done on the basis of population. Seats in the United States House of Representatives, for instance, are reapportioned to individual states every 10 years following a census, with some states that have grown in population gaining seats. The United States Senate, by contrast, is apportioned without regard to population; every state gets exactly two senators. Malapportionment occurs when voters are under or over-represented due to variation in district population.
Gerrymandering is the manipulation of electoral district boundaries for political gain. By creating a few "forfeit" districts where voters vote overwhelmingly for rival candidates, gerrymandering politicians can manufacture more narrow wins among the districts they do seek to win. Gerrymandering relies on the wasted vote effect, effectively concentrating wasted votes among opponents while minimizing wasted votes among supporters. Consequently, gerrymandering is typically done under voting systems using single-member districts, which have more wasted votes.
While much more difficult, gerrymandering can also be done under proportional voting systems when districts elect very few seats. By making three-member districts in regions where a particular group has a slight majority, for instance, gerrymandering politicians can effectively obtain 2/3 of that district's seats. Similarly, by making four-member districts in regions where the same group has slightly less than a majority, gerrymandering politicians can still secure exactly half of the seats. However, any possible gerrymandering that theoretically could occur would be much less effective because minority groups can still elect at least one representative if they make up a significant percentage of the population (e.g. 20-25%). Compare this to single-member districts where 40-49% of the voters can be essentially shut out from any representation.
Sometimes, particularly under non-proportional winner-take-all voting systems, electoral districts can be prone to landslide victories. A safe seat is one that is very unlikely to be won by a rival politician due to the makeup of its constituency. Conversely, a swing seat is one that could easily swing either way. In United Kingdom general elections, the voting in a relatively small number of swing seats usually determines the outcome of the entire election. Many politicians aspire to have safe seats.