An overvote occurs when one votes for more than the maximum number of selections allowed in a contest.[1] The result is a spoilt vote which is not included in the final tally.
One example of an overvote would be voting for two candidates in a single race with the instruction "Vote for not more than one." Robert's Rules of Order notes that such votes are illegal: "If he votes for too many candidates for a given office, however, that particular section of the ballot is illegal, because it is not possible for the tellers to determine for whom the member desired to vote."[2]
Undervotes combined with overvotes (known as residual votes) can be an academic indicator in evaluating the accuracy of a voting system when recording voter intent.[3]
While an overvote in a plurality voting system is always illegal, in certain other electoral methods including approval voting, this style of voting is valid, and thus invalid overvotes are not possible.[4]
In the corporate world, the term "overvote" describes a situation in which someone votes more proxies than they are authorized to, or for more shares than they hold of record.[5]