noun A vote that has been cast but that shows no legally valid preference for a candidate in an election.
noun A vote that has been cast which shows fewer preferences than permitted or expected. (For example, if a ballot allows the voter to vote for mayor and councillor, but the voter has only voted for mayor, leaving the councillor section blank.)
noun The number of such votes in an election.
verb To spoil a ballot by marking it for fewer choices than mandated by the rules.