The counting and survey of every person in a population. In the United States (U.S.), a census is taken every ten years. The census is required by the Constitution for reapportionment and is used in the redistricting process.

Census Block

A census block is the smallest level of geography for which the U.S. Census Bureau releases demographic data. A census block is an area delineated by visible features including roads, streams, railroad tracks as well as nonvisible features such as property lines and city, county, school district limits. Census blocks are contained within Census Tracts.

Census Block Group

A cluster of census blocks within a census tract designated by the U.S. Census Bureau as a subdivision of that census tract.

Census Data

Information and statistics on the U.S. population, gathered by the U.S. Census Bureau and released to the states.

Census Designated Place

A term used by the U.S. Census Bureau to indicate an unincorporated community or a geographical area that has a population but is not legally incorporated. For example, there are unincorporated communities in California that are not part of a city or town but are instead governed by the county where they are located. In Alameda County the unincorporated areas of Ashland, Cherryland, Fairview, Sunol, and Castro Valley are examples of a Census Designated Place.

Census Tract

A unit of census geography defined by the U.S. Census Bureau for the purpose of presenting Decennial Census data. Census tracts are made up of block groups. Their boundaries generally follow visible features, though in some circumstances their boundaries may follow governmental unit boundaries or other non-visible features. In general, census tracts must contain between 1,500 and 8,000 inhabitants.

Citizen Voting Age Population (CVAP)

Refers to residents who are United States citizens and 18 years of age or older.

Coalition District

A coalition district is formed when two or more politically cohesive Minority groups are combined in order to have a meaningful opportunity to elect candidates of their choice. Also known as a Minority Coalition District.

Community of Interest (COI)

A grouping of people concentrated in a geographic area, such as in a city or a neighborhood that share similar political, social, or economic interests. California law defines a COI as a population that shares common social or economic interests that should be included within a single supervisory district for purposes of its effective and fair representation. COI do not include relationships with political parties, incumbents, or political candidates.


A term used to describe a district’s geographic shape. Compactness in redistricting cases often focuses on the regularity or jaggedness of a district boundary and on the extent to which the district’s geographic territory is dispersed from its center. In California law, a district generally is considered compact when “nearby areas of population are not bypassed in favor of more distant populations.


Adjacency. For redistricting purposes, a district is considered to be contiguous if all parts of the district are connected to each other, so that the entire district is within a continuous boundary. Legal standards governing redistricting for various governmental bodies often require all of the territory in each district to be contiguous. Areas that meet only at the points of adjoining corners are not contiguous. Areas that are separated by water and not connected by a bridge, tunnel, or regular ferry service are not contiguous.

County Election Precincts

Geographic units of area established for the purpose of election administration. The voters in an election precinct usually vote at a single polling place, so the votes cast in the precinct may be counted separately from other precincts. See Precincts.

Cracking or Fracturing

A form of voter dilution occurring when districts are drawn so as to divide a geographically compact group of people with shared characteristics so that they fall short of a majority in any district. For example, dividing a minority community into two or more districts. If the minority community is politically cohesive and could elect a preferred candidate if placed in one district, but the minority population is divided into two or more districts where it no longer has electoral control or influence, the voting strength of the minority population is diluted. See Packing.

Crossover District

One in which minority voters do not form a majority, but still reliably control the outcome of the election with some non-minority voters crossing over to vote with the minority group.

Decennial Census

The counting of every person residing in the United States. The decennial census count occurs every ten years and is mandated by the United States Constitution. Decennial census data is used to redraw the lines of voting maps. The decennial census is conducted by the U.S. Census Bureau. See Census.


An expert in statistical analysis related to human populations. Demographers are used in the Redistricting process to analyze Decennial Census data and create the proposed maps.

Deviation and Deviation Range

The amount by which a district’s population differs from the ideal population. The deviation is the amount of population that is less than or greater than the ideal population of a district. In redistricting, a slight deviation may be permissible if based on rational state policies. The redistricting plan’s Deviation Range is the plan’s largest deviation to the plan’s smallest deviation.

Dilution or Vote Dilution

Occurs when the voting strength of a politically cohesive minority group is weakened or watered down by an election system, redistricting plan, or other electoral process or procedure. For example, the creation of districts that either: 1) divide cohesive members of a racial or ethnic minority group among several districts, artificially reducing the group’s opportunity to influence elections (see Cracking or Fracturing) or 2) place extraordinarily high percentages of members of a racial or ethnic minority group in one or more districts, so that minority voting strength is artificially limited to those districts and is minimized in neighboring districts. See Packing.

Equal Protection Clause

See Fourteenth Amendment to the U.S. Constitution.

Fourteenth Amendment to the U.S. Constitution

The U.S. Constitution provision that includes the Equal Protection Clause, which prohibits a state from denying persons equal protection of the law. The Equal Protection Clause is the primary source of the one-person, one-vote principle.

Fifteenth Amendment to the U.S. Constitution

The U.S. Constitution provision providing that the right to vote may not be denied or abridged on account of race or color.


The division of a geographically concentrated cohesive group, such as a racial or political group, among different districts for the purpose of minimizing the group’s voting strength.


See Cracking or Fracturing.

Geographic Information System (GIS)

A graphics-based computer system that relates geographic features (such as census tracts, roads, or county boundaries) to data about those features (such as population or race).


A district or set of districts typically characterized by unusual boundaries, which is drawn to favor one or more individual interest groups over others or to increase the likelihood of a particular political result. Racial gerrymandering is prohibited under federal law and partisan gerrymandering is prohibited under California law.

Homogenous Precinct/Homogenous District

A precinct or district that is nearly all of one race, usually more than 80 percent of one racial group.

Ideal Population

The number of persons to be placed in each district to obtain generally equal population in all districts. The ideal population for each district is obtained by taking the total population of the state, county or jurisdiction and dividing it by the number of districts to be redistricted in the state, county or jurisdiction.

Influence District

Where a minority group constitutes a less than controlling voting group in a district but nevertheless constitutes such a sizeable minority in the district so that it can influence the outcome of an election.

Majority – Minority District

A district where a minority group usually constitutes the majority of voters and can control the outcome of elections in the district.

Method of Equal Proportions

The mathematical formula used, as provided by Federal statute, to reapportion congressional seats among the states after each Decennial Census.

Minority-Coalition District

One in which two or more minority groups combine to form a majority in a district.

One Person, One Vote

The principle that each person’s vote should count equally with every other person’s vote, which is affected by the allocation of the same or substantially the same population to each district of a particular type, such as a congressional district. The courts derive the one-person, one-vote standard primarily from the Equal Protection Clause of the Fourteenth Amendment to the U.S. Constitution. The Fourteenth Amendment also includes the concept that each person in a district (including those not eligible to vote) is entitled to representational equality, that is, to have the same access to the elected representative as each person in every other district.


Drawing district lines in a way that puts as many people with shared characteristics into as few districts as possible. Creating a district with a very high concentration of a particular group of voters, such as a racial or political group, tends to result in the election of the group’s candidate of choice in any election in that district by an overwhelming majority but achieves less overall success by the dilution of the group’s voting strength in a larger number of neighboring districts.

P.L. (Public Law) 94-171

The Federal statute that requires the U.S. Census Bureau to provide, by April 1 of each year following a Decennial Census, the population and race data necessary for redistricting. Because of COVID-19 in 2020, the time period for conducting the Decennial Census was extended by Congress to October 15, 2020, resulting in a later delivery date of the final 2020 census data to states.

Political Subdivision

A division of a state; in California, counties.

Population Estimates

An approximation of the population of a geographic unit at a point in the past or present for which an actual population count is not available.

Population Projections

An approximation of the population of a geographic unit at a point in the future, based on specific assumptions regarding future demographic trends in the geographic unit.


A geographical area created by election officials to group voters in a designated polling place so that an election can be conducted.

Racially-Polarized Voting

The term used to describe circumstances in which the voting preferences of a racial or ethnic group consistently vary from those of other racial or ethnic groups, particularly when the different voting preferences are based on the race of the candidate; also referred to as “racial bloc voting.”


The redistribution of seats in the U.S. House of Representatives based on changes in a state’s population. This occurs so that a state’s representation in the House of Representatives is proportional to its population. Reapportionment is not redistricting, although some states use the terms interchangeably.


The process of redrawing district maps used by political jurisdictions to elect public officials. It applies to all levels of government where district elections are held.


State-adjusted data

Decennial Census data that has been merged with California state election data and adjusted to count persons incarcerated in state prisons in their home communities instead of the communities where correctional facilities where they are detained are located.

Statistical Sampling

The statistical method by which characteristics of a small group are measured and applied to the population as-a-whole.

Topologically Integrated Geographic Encoding and Referencing System (TIGER)

The cartographic map database, prepared by the U.S. Census Bureau, which contains the geographic base units (Census Tracts) that will be used for redistricting.

Total Range of Deviation

The range over which the populations of all districts in a redistricting plan deviate from the ideal, target, or average district population.

Traditional Districting Principles

Factors traditionally used by a state or local jurisdiction to perform redistricting. Examples of traditional redistricting principles may include compactness, contiguity, respect for political subdivisions, and respect for communities of interest.


The error in U.S. Census Bureau data that results from the failure to count some persons or housing units in the Decennial Census. Historically, certain groups, such as members of racial or ethnic minorities, have been disproportionately undercounted by the U.S. Census Bureau.

U.S. Census Bureau

A federal government entity in charge of collecting data about people and the economy. For example, the U.S. Census Bureau counts every resident of the United States during the Decennial Census and collects survey data about the country’s population, housing, and workforce via the American Community Survey.

Voting Age Population

The number of persons in a geographic unit who are at least 18 years of age. Because some population groups, such as racial or ethnic minorities, tend to be younger on average than the population as a whole, the voting age populations are frequently compared in evaluating the potential voting strength of those groups.

Voting Age Citizen Population

The number of persons in a geographic unit who are age 18 plus and who, as citizens, are eligible to vote.

Voting Rights Act

The Federal statute prohibiting discrimination in voting practices on the basis of race or language group, codified as 42 U.S.C. Section 1973 et seq. The official title of the Act is the Voting Rights Act of 1965. The Act was amended in 1982. Section 2, and in some cases Section 5, of the Act are important for redistricting.

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