Definition of challenging adjective from the Oxford Advanced Learner's Dictionary
challenging
adjective
ˈtʃælɪndʒɪŋ ˈtʃælɪndʒɪŋ
1difficult in an interesting way that tests your abilitychallenging work/questions/problemsa challenging and rewarding career as a teacher2done in a way that invites people to disagree or argue with you, or shows that you disagree with themShe gave him a challenging look. ‘Are you really sure?’ she demanded.
Usage note: difficulthardchallengingdemandingtaxingThese words all describe something that is not easy and requires a lot of effort or skill to do.difficult not easy; needing effort or skill to do or understand: The exam questions were quite difficult.◇It is difficult for young people to find jobs around here.hard not easy; needing effort or skill to do or understand: I always found languages quite hard at school.◇It was one of the hardest things I ever did.difficult or hard?Hard is slightly less formal than difficult. It is used particularly in the structure hard to believe/say/find/take, etc., although difficult can also be used in any of these examples.challenging (approving) difficult in an interesting way that tests your ability.demanding difficult to do or deal with and needing a lot of effort, skill or patience: It is a technically demanding piece of music to play.taxing (often used in negative statements) difficult to do and needing a lot of mental or physical effort: This shouldn't be too taxing for you.difficult/hard/challenging/demanding/taxing for somebodydifficult/hard to do somethingphysically difficult/hard/challenging/demanding/taxingtechnically difficult/challenging/demandingmentally/intellectually challenging/demanding/taxing
ˈtʃælɪndʒɪŋ 
ˈtʃælɪndʒɪŋ 
1 difficult in an interesting way that tests your abilitychallenging work/questions/problemsa challenging and rewarding career as a teacher2 done in a way that invites people to disagree or argue with you, or shows that you disagree with themShe gave him a challenging look. ‘Are you really sure?’ she demanded.
Usage note: difficulthard challenging demanding taxingThese words all describe something that is not easy and requires a lot of effort or skill to do.difficult not easy; needing effort or skill to do or understand: The exam questions were quite difficult. ◇ It is difficult for young people to find jobs around here.hard not easy; needing effort or skill to do or understand: I always found languages quite hard at school. ◇ It was one of the hardest things I ever did.difficult or hard?Hard is slightly less formal than difficult. It is used particularly in the structure hard to believe/say/find/take, etc., although difficult can also be used in any of these examples.challenging (approving) difficult in an interesting way that tests your ability.demanding difficult to do or deal with and needing a lot of effort, skill or patience: It is a technically demanding piece of music to play.taxing (often used in negative statements) difficult to do and needing a lot of mental or physical effort: This shouldn't be too taxing for you.difficult/hard/challenging/demanding/taxing for somebodydifficult/hard to do somethingphysically difficult/hard/challenging/demanding/taxingtechnically difficult/challenging/demandingmentally/intellectually challenging/demanding/taxing