Electoral district

An electoral district, also known as an election district, legislative district, voting district, constituency, riding, ward, division, (election) precinct, electoral area, circumscription, or electorate, is a subdivision of a larger state (a country, administrative region, or other polity) created to provide its population with representation in the larger state's legislative body. That body, or the state's constitution or a body established for that purpose, determines each district's boundaries and whether each will be represented by a single member or multiple members. Generally, only voters ("constituents") who reside within the district are permitted to vote in an election held there. District representatives may be elected by a first-past-the-post system, a proportional representative system, or another voting method. They may be selected by a direct election under universal suffrage, an indirect election, or another form of suffrage.
Terminology.
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, especially in British English, but it can also refer to the body of eligible voters or all the residents of the represented area or only those who voted for a certain candidate. The terms "(election) precinct" and "election district" are more common in American English. In Australia and New Zealand, electoral districts are called "electorates", however elsewhere the term "electorate" generally refers specifically to the body of voters.
In India electoral districts are referred to as "Nirvācan Kṣetra" () in Hindi, which can be literally translated to English as "electoral area" though the official English translation for the term is "constituency". The term "Nirvācan Kṣetra" is used while referring to an electoral district in general irrespective of the legislature. When referring to a particular legislative constituency, it is simply referred to as "Kṣetra" along with the name of the legislature, in Hindi (e.g. 'Lok Sabha Kshetra' for a Lok Sabha constituency). Electoral districts for municipal or other local bodies are called "wards".
In Canada, districts are colloquially called in English "ridings" (stemming from an earlier British geographical subdivision) (in some parts of Canada "constituencies" is used for provincial districts and "ridings" for federal districts), and in French, "circumscription" or (colloquially) "comté", "county". Local electoral districts are sometimes called "wards", a term also used for administrative subdivisions of a municipality. 
In local government in the Republic of Ireland voting districts are called "electoral areas".
District magnitude.
"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" – historically in the US especially called a plural district – has more than one. Voting systems that seek or are called proportional representation entail multi-member districts – or leveling seats. Levelling seats are rarely authorized to foist away a victory from a plurality-winning candidate; most are instead additional members for the entire set of districts, at-large. 
In voting systems other than general ticket, plurality block voting and certain pro-landslide party-list systems, a Droop quota or threshold is recognised. This means the higher the district magnitude, the more proportional the system (and the greater the number of distinct parties or choices that can be represented). A compromise between the two, a heavily modified general ticket, is the teams model. This is the system of the numerically dominant Group Representation Constituencies in Singapore which requires one team member (at least) to be of a different race from the others.
In proportional representation, in a state where civil and political rights are liberally exercised, there exist political competing views evolving into parties or independent candidatures, a diverse mainstream media, and a moderate cap on election expenses, greater magnitude diversifies the party or non-affiliated makeup of the elected body. This diversity of winners enables clear representation of minorities in viewpoint; a 10%-polling minority party in a district where turnout is quite uniform will not win a seat in a 5-member district but if its turnout is in line with the others will do so in a 9-member district as this is the minimum to exceed a Droop quota.
The distribution of minority groups (parties) interplays with the magnitude: an unpopular or non-campaigning party, body-wide, tends to secure a seat if they are concentrated in a district, and require less concentration if the electoral district's magnitude is greater. Likewise if their support is significant but very diffuse they are more likely to win seats if the magnitude is greater. District magnitude can vary. Examples: 
Maximum magnitude:
Apportionment and redistricting.
"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 after every census while Belgium uses its existing administrative boundaries for electoral districts and instead modifies the number of representatives allotted to each. Israel and the Netherlands are among the few countries that avoid the need for apportionment entirely by electing legislators at-large.
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. By contrast, seats in the Cantonal Council of Zürich are reapportioned in every election based on the number of votes cast in each district, which is only made possible by use of multi-member districts, and the House of Peoples of Bosnia and Herzegovina, by contrast, is apportioned without regard to population; the three major ethnic groups - Bosniaks, Serbs, and Croats - each get exactly five members. "Malapportionment" occurs when voters are under- or over-represented due to variation in district population.
In some places, geographical area is allowed to affect apportionment, with rural areas with sparse populations allocated more seats per elector: for example in Iceland, the Falkland Islands, Scottish islands, and (partly) in US Senate elections.
Gerrymandering.
"Gerrymandering" is the manipulation of electoral district boundaries for political gain. By creating a few "forfeit" districts where opposing candidates win overwhelmingly, gerrymandering politicians can manufacture more, but narrower, wins for themselves and their party. 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 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%), compared to single-member districts where 40-49% of the voters can be essentially shut out from any representation
Swing seats and safe seats.
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 and United States presidential and congressional 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. 
In large multi-party systems like India, swing seats can lead to a hung assembly like situation if a significant number of seats go for regional parties instead of the larger national parties who are the main competitors at the national or state level, as was the situation in the Lok Sabha (Lower house of the Parliament of India) during the 1990s.
Constituency work.
Elected representatives may spend much of the time serving the needs or demands of individual "constituents", meaning either voters or residents of their district. This is more common in assemblies with many single-member or small districts than those with fewer, larger districts. In a looser sense, corporations and other such organizations can be referred to as constituents, if they have a significant presence in an area.
Many assemblies allow free postage (through franking privilege or prepaid envelopes) from a representative to a constituent, and often free telecommunications. Caseworkers may be employed by representatives to assist constituents with problems. Members of the U.S. Congress (both Representatives and Senators) working in Washington, D.C. have a governmentally staffed district office to aid in constituent services. Many state legislatures have followed suit. Likewise, British MPs use their Parliamentary staffing allowance to appoint staff for constituency casework. Client politics and pork barrel politics are associated with constituency work.
Special constituencies with additional membership requirements.
In some elected assemblies, some or all constituencies may group voters based on some criterion other than, or in addition to, the location they live. Examples include:
Voting without constituencies.
Not all democratic political systems use separate districts or other electoral subdivisions to conduct elections. Israel, for instance, conducts parliamentary elections as a single district. While the 26 electoral districts in Italy and the 20 in the Netherlands have a role in the actual election, but no role whatsoever in the division of the seats. Ukraine elected half of the Verkhovna Rada (the Ukrainian Parliament) in this way in the elections in October 2012.