Definitions of challenge:

• noun:   a demand by a sentry for a password or identification
• noun:   questioning a statement and demanding an explanation

  Example: "His challenge of the assumption that Japan is still our enemy"

• noun:   a formal objection to the selection of a particular person as a juror
• noun:   a call to engage in a contest or fight
• noun:   a demanding or stimulating situation

  Example: "They reacted irrationally to the challenge of Russian power"

• verb:   raise a formal objection in a court of law
• verb:   ask for identification

  Example: "The illegal immigrant was challenged by the border guard"

• verb:   issue a challenge to

  Example: "Fischer challenged Spassky to a match"

• verb:   take exception to

  Example: "She challenged his claims"