Definition - What does Referendum mean?
A referendum is a process whereby registered voters approve or reject proposed law(s). This type of vote is authorized under the state constitution and is generally, but not always, held in conjunction with a local or state election.
A national referendum—in which all eligible voters can cast ballots with regard to proposed law(s)—is not permitted in the United States.
Justipedia explains Referendum
There are different types of referendums: "mandatory," "optional," "popular" and "legislative."
A mandatory referendum is one that must be held in accordance with the rules set forth in a state constitution. Voters in these circumstances are asked to approve or reject changes to the state constitution, or the issuance of bonds. An example with regard to the latter would be a case in which a school district wants to issue bonds for the construction of a new building.
On the other hand, an optional referendum is one in which the voters are tasked with approving or denying routine measures.
The third type, the popular referendum, is permitted in only some states. This type of referendum differs from the others because it allows eligible participants to voice their opinions by creating and signing petitions instead of casting ballots in an election.
Finally, the legislative referendum is one in which voters approve or reject matters put forth by state lawmakers, or state or local agencies.