challenging adj.
VERBS be, prove | make sth We have changed the course to make it more challenging. | find sth He found the course academically challenging.
ADV. extremely, very | quite | enough, sufficiently The tasks were not challenging enough for me and I got bored. | academically, intellectually, politically
You can also check other dicts: challenging (English, 中文解释 ), wordnet sense, Collins Definition