A direct popular vote on a specific question, such as a law, constitutional amendment or policy. It may be binding or advisory and may be called by authorities, triggered by citizens or required by a constitution.
In more detail
Switzerland's constitution requires a referendum on constitutional amendments, which need a majority of both people and cantons, and allows an optional referendum on federal laws if 50,000 signatures are gathered within 100 days. Elsewhere referendums are rarer and often advisory. Whether they strengthen democracy or let majorities override minorities and complex trade-offs is debated.