undervote

/AH1 N D ER0 V OW1 T/
noun
  1. 1

    A vote that has been cast but that shows no legally valid preference for a candidate in an election.

  2. 2

    The number of such votes in an election.

verb
  1. 1

    To spoil a ballot by marking it for fewer choices than mandated by the rules.

Source: https://en.wiktionary.org/wiki/undervote