1. Occurs when the number of voter selections in a contest is less than the maximum number allowed for that contest or when no selection is made. The number of undervotes is equal to the number of votes lost, for example, if no selection is made in a vote for two contest the number of votes lost is two.
Source: Election Terminology Glossary - Draft, National Institute of Standards and Technology (NIST), https://pages.nist.gov/ElectionGlossary/; Glossary of terms database, U.S. Election Assistance Commission, https://www.eac.gov/glossary/
2. An undervote occurs when a voter makes fewer selections than she is entitled to make. For example, voting for only two candidates when the voter is entitled to vote for three out of seven candidates is an undervote.
Source: COUNTING VOTES 2012: A State by State Look at Voting Technology Preparedness, Verified Voting Foundation, Rutgers School of Law - Newark Constitutional Litigation Clinic & Common Cause Education Fund, https://countingvotes.org/sites/default/files/CountingVotes2012_Final_August2012.pdf
3. The lack of indication on a cast ballot about the voter's choice for a given contest. Undervotes are legal, because there is no requirement that a voter must vote on every contest, but may or may not reflect the actual intention of the voter in casting (or not casting) a vote for the contest in question.
Source: Asking the Right Questions about Electronic Voting, National Research Council on the National Academies, https://www.nap.edu/catalog/11449/asking-the-right-questions-about-electronic-voting
4. An undervote is a vote for fewer choices than permitted, such as not voting for President. An undervote may or may not be an error. A voter might have tried to vote for a candidate but failed to mark the ballot unambiguously or might have chosen not to vote for any candidate for a particular office.
Source: ELECTIONS: The Nation’s Evolving Election System as Reflected in the November 2004 General Election, U.S. Government Accountability Office, https://www.gao.gov/assets/160/157713.pdf
5. Marking the ballot for no candidate, or fewer than the maximum number allowed in the race; where only one vote was permitted, this results in the ballot being rejected; where multiple choices are permitted, the valid markings are still recorded; often this occurs on purpose to indicate a protest vote, but can also occur unintentionally.
Source: Independent Panel on Internet Voting, British Columbia, https://elections.bc.ca/docs/recommendations-report.pdf