A call to engage in a contest, fight, or competition: a challenge to a duel. An act or statement of defiance; a call to confrontation: a challenge to the government's authority. A demand for explanation or justification; a calling into question: a challenge to a theory. A sentry's call to an unknown party for proper identification. A test of one's abilities or resources in a demanding but stimulating undertaking: a career that offers a challenge. A claim that a vote is invalid or that a voter is unqualified. Law A formal objection to the inclusion of a prospective juror in a jury. A legal case testing the validity of an action taken, particularly by the government. Immunology The induction or evaluation of an immune response in an organism by administration of a specific antigen to which it has been sensitized. To call to engage in a contest, fight, or competition: challenged me to a game of chess. To invite with defiance; dare: challenged him to contradict her. To confront or struggle with (something) as a test of one's abilities: rafters challenging the rapids. To take exception to; call into question; dispute: a book that challenges established beliefs. To order to halt and be identified, as by a sentry. Law To take formal objection to (a prospective juror). To bring a legal case testing the validity of an action, particularly by the government. To question the qualifications of (a voter) or the validity of (a vote). To have due claim to; call for: events that challenge our attention. To summon to action, effort, or use; stimulate: a problem that challenges the imagination. Immunology To induce or evaluate an immune response in (an organism) by administering a specific antigen to which it has been sensitized. To make or give voice to a challenge. To begin barking upon picking up the scent. Used of hunting dogs.