Single-winner methods are voting methods that elect only one winner.
(a.k.a. ranked choice voting, Hare Method, preferential voting)
Despite being a method where voters rank their choices, IRV is confused with approval voting surprisingly often. IRV has voters rank their preferences. If a candidate is ranked first on more than 50% of the remaining valid ballots, then that candidate wins. If not, the candidate with the least first-choice votes is eliminated and those ballots are transferred to the other candidates. This process repeats until a candidate has more than 50% of the remaining valid ballots.
Compare this to approval voting where there is no ranking. It’s simply the candidate approved/chosen the most wins. Approval voting also performs better.
“Modified Cumulative Voting”
People sometimes unfortunately employ a variant of cumulative voting, in which there is only one winner but multiple votes. For instance, you are given three votes. You may distribute them however you like. If you want to give them all to the same candidate, you can. Note that if this were approval voting, then you would never be limited to the number of candidates you could choose, and you would not be allowed to choose a candidate multiple times.
The problem is that this system degenerates into ordinary plurality voting. It fails the favorite betrayal criterion, meaning it can hurt you to vote for your sincere favorite candidate. Say you prefer the Green Party candidate, but the Democrat and Republican are the clear frontrunners. Then your best strategy is to vote for your favorite between the frontrunners. You “betray” the Green, so to speak. And your best tactic is to give all of your votes to the same candidate.
Another option sometimes instituted by people without a background in electoral system design is to allow more votes than the number of winners. E.g. there is one winner, but you may vote for three candidates — but, unlike cumulative voting, not more than once for the same candidate. Notice that this is the opposite of limited voting. Also note that if this were approval voting, then voters would not be limited whatsoever in the number of candidates that they could select.