A ballot measure or ballot proposition is an opportunity for voters in an election to have a say, binding or non-binding, on what the law should be.
Ballot measures are often discussed in the voting reform community in the context of passing ballot measures, as it's believed that passing voting reforms through legislatures is difficult due to politicians' self-interest in keeping the system that elected them.
The usual method of voting on a ballot measure is to ask voters to vote Yes or No, with the majority prevailing.
Other methods of doing ballot measures have been discussed, however. For example, Approval voting has been used before, and Score voting allows voters to express strength of preference while still, if the measure has only two options to vote on, allowing the majority to force their preference if they wish. See Maximize Affirmed Majorities#MAM voting on ballot propositions for an example of a Condorcet method used for ballot measures.