challenge

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun Accusation; charge.
• noun A claim or demand; pretension.
• noun A summons or invitation to a duel; a calling upon one to engage in single combat, as for the vindication of the challenger's honor; a defiance.
• noun Hence An invitation to a contest or trial of any kind: as, a challenge to a rubber at whist; a challenge to a public debate; “a challenge to controversy,”
• noun The letter or message containing the summons to a combat or contest.
• noun Milit., the act of a sentry in demanding the countersign from any one who approaches his post.
• noun In hunting, the opening cry of hounds on first finding the scent of their game.
• noun A calling in question; an exception taken, as to the tenability of a proposition, or a person's right to do something or to hold something.
• noun In law, an objection to a juror; the claim of a party that a certain juror shall not sit in the cause.
• noun In the East Indies, an exception taken by a ryot to the assessment of a neighbor's holding when it is less than that of his own poorer holding, accompanied by an offer to take over the neighbor's holding at a higher assessment, and a claim for the assessment on his own to be correspondingly reduced.
• To accuse; call to answer; censure.
• To lay claim to; demand as due or as a right: as, the Supreme Being challenges our reverence and homage.
• To call, invite, or summon to single combat or duel.
• To call to a contest; call into opposing activity; invite to a trial; defy: as, to challenge a man to prove what he asserts (implying defiance).
• To take exception to; object to (a person or thing); call in question: as, to challenge the accuracy of a statement. Specifically
• In law, to object or take exception to, as a juror or jury panel. See challenge, n., 9—7. Milit., to demand the countersign from: as, a sentry is bound to challenge every person appearing near his post. See challenge, n., 6.
• In hunting, to whimper or cry when the scent of game is first discovered: said of a hound.

from the GNU version of the Collaborative International Dictionary of English.

• noun An invitation to engage in a contest or controversy of any kind; a defiance; specifically, a summons to fight a duel; also, the letter or message conveying the summons.
• noun The act of a sentry in halting any one who appears at his post, and demanding the countersign.
• noun obsolete A claim or demand.
• noun (Hunting) The opening and crying of hounds at first finding the scent of their game.
• noun (Law) An exception to a juror or to a member of a court martial, coupled with a demand that he should be held incompetent to act; the claim of a party that a certain person or persons shall not sit in trial upon him or his cause.
• noun United States An exception to a person as not legally qualified to vote. The challenge must be made when the ballot is offered.
• noun (Law) an exception to the whole panel.