a hollow gesture, IMHO
A vote for a third-party candidate made not to elect that candidate but to indicate displeasure with the candidates of the two major political parties.
A vote for a third- or minor-party candidate made without much hope of electing that candidate but intended to indicate displeasure with the candidates of the two major political parties.
a vote for a third party candidate (who is not likely to win) that is meant to show displeasure with the mainstream candidates or parties.
A Protest vote is a vote cast in an election to demonstrate the caster's unhappiness with the choice of candidates or refusal of the current political system. It can thus be said "conjectural," as the voter would accept others candidates in the same system, or "structural," if the voter is opposed to the whole system — usually representative democracy, but it may also signify opposition to a two-party system where "third options" are always rejected. In this latter case, protest vote may take the form of a legal vote, but instead of voting for the mainstream candidates, it is a vote in favor of a minority or fringe candidate, either from the far-left, far-right or self-presenting as a candidate foreign to the political system (a gesture which might be interpreted by political analysts as a form of populism, as it is an appeal to the "people" against the "establishment").