challenge >> /ˈtʃæləndʒ/
Definition: [transitive verb] Invite (someone) to engage in a contest.
Example: he challenged one of my men to a duel
Definition: [transitive verb] Dispute the truth or validity of.
Definition: [transitive verb] Administer an immunogenic or infectious agent to an animal or person, in order to study the resulting immune response or measure the efficacy of a vaccine.
Definition: [noun] A call to take part in a contest or competition, especially a duel.
Example: he accepted the challenge
Definition: [noun] An objection or query as to the truth of something, often with an implicit demand for proof.
Example: a challenge to the legality of the order
Definition: [noun] The administration of an immunogenic or infectious agent to an animal or person, in order to study the resulting immune response or measure the efficacy of a vaccine.
Example: challenge trials have yielded positive results when carefully and ethically conducted