1. a call or summons to engage in any contest, as of skill, strength, etc.
2. something that by its nature or character serves as a call to battle, contest, special effort, etc.: Space exploration offers a challenge to humankind.
3. a call to fight, as a battle, a duel, etc.
4. a demand to explain, justify, etc.: a challenge to the treasurer to itemize expenditures.
5. difficulty in a job or undertaking that is stimulating to one engaged in it.
6. the demand of a sentry for identification or a countersign.
7. a formal objection to the qualifications of a particular juror, to that juror serving, or to the legality of an entire jury.
8. the assertion that a vote is invalid or that a voter is not legally qualified.
9. the process of inducing or assessing physiological or immunological activity by exposing an organism to a specific substance.
10. the crying of a hound on finding a scent.

11. to summon to a contest of skill, strength, etc.
12. to take exception to; call in question: to challenge the wisdom of a procedure.
13. to demand as something due or rightful.
14. to halt and demand identification or countersign from.
15. to take formal exception to (a juror or jury).
16. to have a claim to; invite; arouse; stimulate: a matter which challenges attention.
17. to assert that (a vote) is invalid.
18. to assert that (a voter) is not qualified to vote.
19. to expose an organism to a specific substance in order to assess its physiological or immunological activity.
20. to lay claim to.

21. to make or issue a challenge.
22. (of hounds) to cry or give tongue on picking up the scent.

23. donated or given by a private, corporate, or government benefactor on condition that the recipient raise an additional specified amount from the public: a challenge grant.

1. to invite or summon (someone to do something, esp to take part in a contest)
2. to call (something) into question; dispute
3. to make demands on; stimulate
4. to order (a person) to halt and be identified or to give a password
5. to make formal objection to (a juror or jury)
6. to lay claim to (attention, etc)
7. (of a hound) to cry out on first encountering the scent of a quarry
8. to inject (an experimental animal immunized with a test substance) with disease microorganisms to test for immunity to the disease

9. a call to engage in a fight, argument, or contest
10. a questioning of a statement or fact; a demand for justification or explanation
11. a demanding or stimulating situation, career, object, etc
12. a demand by a sentry, watchman, etc, for identification or a password
13. an assertion that a person is not entitled to vote or that a vote is invalid
14. a formal objection to a person selected to serve on a jury (challenge to the polls) or to the whole body of jurors (challenge to the array)

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