Selecting more than one candidate on an election ballot.
Election officials who use punch card systems similar to the one in Palm Beach County expect that a certain number of ballots will have more than one hole punched in a given race, a practice called "overvoting."
—Don Van Natta, Jr, and Dana Canedy, "Florida Democrats Say Ballot's Design Hurt Gore," The New York Times, November 9, 2000
Thus, with a voting machine there's no chance of your accidentally overvoting the ticket and spoiling your ballot so that it would be thrown out and your vote not counted.
—"What good are voting moachines?," Kiplinger's Personal Finance, June 1, 1967
Overvoting isn't a new word (a quick check showed it being used at least as far back as 1967), but it's definitely topical! (Note, too, that you see over-voting at least as much as overvoting.) There's even an overvote (verb) in the Oxford English Dictionary from 1641, but it's an obsolete word that means "to defeat by a majority of votes."
The opposite of overvoting is, of course, undervoting, which means not choosing any candidate on an election ballot.
My favorite voter quote so far:
"I did find [the Palm Beach ballot] confusing and I'm a member of Mensa."