any voting system that allows many voters to express formal disapproval simultaneously, in a system where they all share some power. Unlike most voting systems, it requires that only negative measures or choices be presented to the voter or representative. If used to select candidates for an office, or for continuation to a next round of voting or play, it is either single- or multi-winner, as everyone who is not disapproved of is in effect a winner, for that round.
Besides the occasional recall election, we do not have disapproval voting in the US. The Wikipedia page notes that in Russia, you can vote "against all" candidates, and in Canada you can decline the ballot (which doesn't count as a vote, but is tallied separately. With a large proportion of the population voting for the lesser of two evils, would it be unconstitutional or against any congressional law for a state to adopt a disapproval voting system instead of a first past the post system?