CHALLENGE - [noun] a difficult situation in which you have to make great (mental or physical) effort to deal with it
Examples of use
It was a very difficult task but she rose to the challenge.
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her