• Noun: a demanding or stimulating situation; example: "they reacted irrationally to the challenge of Russian power"
• Noun: a call to engage in a contest or fight
• 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 demand by a sentry for a password or identification
• Verb: take exception to; example: "She challenged his claims"
• Verb: issue a challenge to; example: "Fischer challenged Spassky to a match"
• Verb: ask for identification; example: "The illegal immigrant was challenged by the border guard"
• Verb: raise a formal objection in a court of law
And obviously, from our own personal point of view, the principal challenge is a personal challenge.