Definition of blackball

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black·ball    (blkbôl) KEY 

NOUN:

1. A negative vote, especially one that blocks the admission of an applicant to an organization.
2. A small black ball used as a negative ballot.

TRANSITIVE VERB:
black·balled, black·ball·ing, black·balls

1. To vote against, especially to veto the admission of.
2. To shut out from social or commercial participation; ostracize or boycott.

OTHER FORMS:
blackballer(Noun)

SYNONYMS:
blackball, blacklist, boycott, ostracize

These verbs mean to exclude from social, professional, or commercial activities: blackballed by the fraternity; blacklisted because of her political beliefs; a threat to boycott the product; ostracized following the harassment charges.
Antonym: admit