Additional member system (AMS)
Another term for a mixed member proportional voting system.

Alternative vote
Another term for instant run-off voting.

Approval voting
A voting system used in single-member district systems and single-office elections in which voters can vote for, or approve of, as many candidates as they wish. Each candidate approved of receives one vote and the candidate with the most votes wins. The winner need not garner a majority of the votes.

At-large system
Another term for a multi-member plurality voting system.

Bullet voting
The term for when a voter votes for only one candidate, when he or she had the option of voting for more than one.

Choice voting
A proportional representation system in which voters rank the candidates on the ballot, putting a 1 next to their first choice, a 2 next to their second choice, and so on. Candidates receiving votes beyond the quota needed to get elected are declared winners. Voters’ ballots are re-allocated to their next preferences when their first candidate is eliminated or when there are surplus votes for an elected candidate.

Closed List
A type of ballot found in list proportional representation in which votes are cast for a party and it’s list of candidates. The order of the candidates on the list is fixed and it is determined by the party, usually in a convention, caucus or primary. Voters cannot express a preference for any particular candidate within that list.

Compensatory seats
The list PR seats in a mixed member proportional system that are allocated to parties on the basis of their proportion of the vote. These seats are designed to compensate for any disproportionality in representation resulting from the single-member district portion of the elections.

Cumulative vote (CV)
A system that uses multimember districts and in which voters have the same number of votes as there are seats being contested. Voters may allocate their votes among the various candidates in any way they see fit – including giving more than one vote to a particular candidate. In a three seat district, a voter may give two votes to one candidate and one to another; or one vote each to three candidates, and so on. Considered a semi-proportional system.

Districts
The geographical regions into which a city, state, or country is divided for election purposes. Single-member districts elect one member of the legislature. Multimember districts elect two or more members.

District magnitude
The number of seats elected in a district.

Electoral System
Also called a "voting system," this is the set of procedures that determine how people are elected to office.  These procedures include how the ballot is structured, how people cast their votes, how those votes are counted, and how the winners are determined.  Should not be confused with the term "election system" which is a broader term that refers to all the procedures involved in elections, including ballot access, campaign finance laws, etc.

First past the post system (FPTP)
A British term that refers to single-member district plurality elections.

Gerrymandering
The manipulation of district boundary lines in order to unfairly advantage or disadvantage a candidate or political group. Typically used to create a district that is favorable to an incumbent, or a series of districts that allows a particular party or political group to receive more seats than it deserves based in its proportion of the vote.

Instant run-off voting (IRV)
A majority voting system used in single-member district systems and single-office elections. Voters mark their preferences on the ballot by putting a 1 next to their first choice, a 2 next to their second choice, and so on. A candidate who receives over 50% of the first preference votes is declared the winner. Otherwise, the weakest candidate is eliminated and their votes are reallocated to the voters’ second choices. This reallocation process continues until one candidate receives a majority of the votes.

Invalid votes
Another term for spoiled ballots.

Limited vote (LV)
A plurality system that uses multimember districts and in which voters have more than one vote, but fewer votes than the number of seats being contested. For example, there might be five seats to fill, but voters have only 3 votes to cast. The candidates with the most votes are declared the winners. Considered a semi-proportional system.

List proportional representation (List PR)
A system that uses multimember districts and in which voters choose between lists of candidates offered by political parties. The seats in the district are distributed among the parties according to their proportion of the vote. The two basic forms are closed list and open list PR.

Majority system
A single-member district system or single office election system that tries to ensure that the winning candidates receives an absolute majority of the votes. Examples include the two ballot run-off system and instant run-off voting.

Majority sweep
An election result where the party with the majority of votes wins all the seats in a district.

Manufactured majority
A situation where a single party wins less than 50% of the vote, but receives over 50% of the seats in the legislature. For example, a party’s candidates might receive only 45% of the vote, but win 55% of the legislative seats.

Minority rule
An election result where the party that comes in second place in the voting wins the majority of seats in the district or the legislature. For example, party A might get 48% of the vote and 55% of the seats, while party B gets 52% of the votes and 45% of the seats.

Mixed member proportional (MMP)
A PR system that combines list PR and single-member plurality districts. In the German variant, one half of the legislators are elected in single member districts. The other half of the seats are filled from the party lists, and they are added on to the number of districts seats that a party wins so that their total share of the legislative seats is equivalent to the proportion of the votes won by that party on the list portion of the ballot.

Multimember district
A district in which more than one member is elected to the legislature.

Multimember Plurality Voting System
A system where candidates run in large multimember districts and voters have as many votes as there are candidates to be elected. Used almost exclusively in town and city elections in the United States. Also know as at-large voting.

Open List
A type of ballot found in list proportional representation in which voters can express a preference for a candidate within a party list as well as vote for that list. Votes for individual candidates improve their chances of being elected.

Parallel system
A mixed system in which some legislative seats are filled in single-member districts contests and others from a list PR election. Unlike the mixed member proportional system, however, there is no effort to use the list seats to produce an overall proportional outcome in the legislature. Since the list seats do not fully correct the disproportionality caused by the single-member district contests, this is considered a semi-proportional system.

Plurality system
Voting systems which use single or multi-member districts and in which the winner is the candidate or candidates with the most votes. The most common in the U.S. is the single-member district plurality system.

Preference Voting
Another term for choice voting.

Proportional representation
A group of voting systems whose major goal is to ensure that parties and political groups are allocated seats in legislative bodies in proportion to their share of the vote. So a party receiving 30% of the national vote should receive 30% of the seats in the national legislature.

Quota
Another term for threshold – the minimum proportion of the vote needed to receive any seats in a PR system.

Semi-proportional system
Voting systems that may produce more proportional representation than plurality/majority systems, but that usually fall short of the fully proportional results produced by PR systems like list PR, choice voting, and mixed member proportional systems. The three most common kinds of semi-proportional systems are cumulative voting, limited voting, and parallel voting.

Sincere voting
When voters are able to cast their votes for their most preferred candidate. This contrasts with "insincere" or "strategic" voting where voters must cast a vote for a candidate other than their first preference in order to best pursue their political interests,

Single non-transferable vote (SNTV)
This voting system is a variation of the limited vote and it belongs to the family of semi-proportional systems. In this system, there are multiple seats at stake in the district, but voters are limited to one vote.

Single-member district
A district in which only one member is elected to the legislature.

Single-member plurality voting (SMP)
A system in which candidates are elected in single-member districts, with the winner being the one with the most votes – the plurality of the votes.

Single transferable vote (STV)
Another term for choice voting, usually found in the academic literature.

Single office election
An election to choose the occupant of a single office, such as mayor or governor – in contrast to elections which choose entire legislatures.

Spoiled ballots
Ballots which, because of accidental or deliberate errors in the marking process, are declared invalid and are eliminated from the count.

Spoiler
A phenomenon of plurality/majority voting systems where an independent or third party candidate takes enough votes away from one major party candidate to ensure the victory of the other major party candidate, who would not have won otherwise.

Threshold
In a proportional representation system, the minimum portion of the vote that a party must receive in order to receive any seats in the legislature. Known technically as the threshold of exclusion because if a party reaches this threshold it cannot be excluded from winning a seat.

Two-Round System (TRS)
A majority voting system that is used in single-member district systems and single-office elections to ensure that the winner has the support of the majority of the voters. If no candidate receives a majority of the vote in the first round of voting, a second election is held a short time later. In this second election, the top two candidates face off and one of them usually wins with a majority of the vote. Sometimes also called the "run-off system" and the "second ballot system."

Voting System
The set of procedures that determine how people are elected to office.  These procedures include how the ballot is structured, how people cast their votes, how those votes are counted, and how the winners are determined.

Wasted votes
Votes that do not ultimately contribute towards the election of particular candidate. Votes that are cast but that do not actually produce any representation.

Winner-take-all systems
Another term for a plurality or majority voting system that produces one winner in each district.

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