The optional referendum (German: fakultatives Referendum; French: référendum facultatif, Italian: referendum facoltativo, Romansh: referendum facultativ) is an instrument of direct democracy in Switzerland. It allows citizens to oppose laws voted by the federal parliament, cantonal and/or municipal decrees by legislative and/or executive bodies.
On a federal level a vote will be organised on every law against which opponent collect 50,000 valid signatures during the period of 100 days after publication by the parliament.
A referendum can also be requested by a minimum of eight cantons, the so-called cantonal referendum (not to be confused with a mandatory or optional referendum on a cantonal level).
It is different from a mandatory referendum in that a collection of signatures is necessary to organise the referendum.
After pressure by a grass-roots movement, the optional referendum was introduced in 1874.