Definition: a public vote on a particular issue
Latin has given English a sizable portion of its vocabulary, and one of the reasons that so many of our words are descended from that language is that they have entered our tongue at a number of distinct points. Some, such as butter, date back to the Roman invasion of Britain. Others came to us in the middle ages, from the Norman Conquest. Others still did not arrive until the 19th century, often as part of an expanding scientific vocabulary. Referendum is one of those late arrivals, initially used to describe a vote on the Swiss constitution at that time. Both referendums and referenda are correct.
Of course it will all take time—
The population must be reassured,
The boundary must be surveyed,
There'll be royal commissions, referenda….
—Margaretta D’Arcy and John Arden, The Non-Stop Connolly Show: Part Six, 1978 quote here