An overvote occurs when one votes for more than the maximum number of selections allowed in a contest.
[1] The result is a spoiled vote which is not included in the final tally.
Robert's Rules of Order notes that such votes are illegal.
[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.
[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.