WordWeb Online

1. Requiring full use of your abilities or resources
   "performed the most challenging task without a mistake";
   - ambitious
    
2. Stimulating interest or thought
   "a challenging hypothesis";
   - thought-provoking
    
3. Disturbingly provocative
   - intriguing

1. Question the truth or validity of; take exception to
   "She challenged his claims";
   - dispute, gainsay
    
2. Issue a challenge to
   "Fischer challenged Spassky to a match"
    
3. Ask for identification
   "The illegal immigrant was challenged by the border guard"
    
4. (law) raise a formal objection in a court of law
   - take exception

See also: difficult, hard, provocative, stimulating

Type of: ask for, bespeak, bid, call for, contend, contest, invite, object, quest, repugn, request

Encyclopedia: Challenging