Referendum

A referendum (plural referendums, see below) is a direct vote in which an entire electorate is asked to vote on a particular proposal. This may result in the adoption of a new law. In some countries it is synonymous with a plebiscite or a vote on a ballot question.

Some definitions of 'plebiscite' suggest that it is a type of vote to change the constitution or government of a country. However, some other countries define it differently. For example, Australia defines 'referendum' as a vote to change the constitution, and 'plebiscite' as a vote that does not affect the constitution. In Ireland, the vote to adopt its constitution was called a "plebiscite", but a subsequent vote to amend the constitution is called a 'referendum', and so is a poll of the electorate on a non-constitutional bill.

'Referendum' is the gerundive form of the Latin verb refero, literally "to carry back" (from the verb fero, "to bear, bring, carry" plus the inseparable preposition 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 qui 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 neccessity or compulsion, that which "must" be done, rather than that which is "fit for" doing). Its use as a noun in English is thus not a strictly grammatical usage of a foreign word, but is rather a freshly coined English noun, which therefore 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 thus insupportable according to the rules of both Latin and English grammar alike. 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".

...
Wikipedia