A referendum, plebiscite, or ballot measure is a direct vote by the electorate (rather than their representatives) on a proposal, law, or political issue. A referendum may be either binding (resulting in the adoption of a new policy) or advisory (functioning like a large-scale opinion poll).

'Referendum' is the gerundive form of the Latin verb referre, literally "to carry back" (from the verb ferre, "to bear, bring, carry" plus the inseparable prefix re-, here meaning "back"). As a gerundive is an adjective, not a noun, it cannot be used alone in Latin, and must be contained within a context attached to a noun such as Propositum quod referendum est populo, "A proposal which must be carried back to the people". The addition of the verb sum (3rd person singular, est) to a gerundive, denotes the idea of necessity or compulsion, that which "must" be done, rather than that which is "fit for" doing. Its use as a noun in English is not considered a strictly grammatical usage of a foreign word but is rather a newly coined English noun, which follows English grammatical usage, not Latin grammatical usage. This determines the form of the plural in English, which according to English grammar should be "referendums". The use of "referenda" as a plural form in English (treating it as a Latin word and attempting to apply to it the rules of Latin grammar) is unsupportable according to the rules of both Latin and English grammar. The use of "referenda" as a plural form is posited hypothetically as either a gerund or a gerundive by the Oxford English Dictionary, which rules out such usage in both cases as follows:

Referendums is logically preferable as a plural form meaning 'ballots on one issue' (as a Latin gerund, referendum has no plural). The Latin plural gerundive 'referenda', meaning 'things to be referred', necessarily connotes a plurality of issues.

It is closely related to agenda, "those matters which must be driven forward", from ago, to impel or drive forwards; and memorandum, "that matter which must be remembered", from memoro, to call to mind, corrigenda, from rego, to rule, make straight, those things which must be made straight (corrected), etc.

The term 'plebiscite' has a generally similar meaning in modern usage and comes from the Latin plebiscita, which originally meant a decree of the Concilium Plebis (Plebeian Council), the popular assembly of the Roman Republic. Today, a referendum can also often be referred to as a plebiscite, but in some countries the two terms are used differently to refer to votes with differing types of legal consequences.

In Australia, a 'referendum' is often said to be a vote to change the federal constitution and 'plebiscite' a vote which does not affect the federal constitution. However, this is erroneous as not all federal referendums have been on constitutional matters (such as the 1916 Australian conscription referendum), and state votes that likewise do not affect either the federal or state constitution are frequently said to be referendums (such as the 2009 Western Australian daylight saving referendum). Historically, they are used by Australians interchangeably and a plebiscite was considered another name for a referendum.

In Ireland, 'plebiscite' referred to the vote to adopt its constitution, but a subsequent vote to amend the constitution is called a 'referendum', as is a poll of the electorate on a non-constitutional bill.

The name and use of the 'referendum' is thought to have originated in the Swiss canton of Graubünden as early as the 16th century.