ASSERTION - [noun] a forceful statement or claim, behaving in a determined way
Examples of use
She went on repeating her assertion that she was not guilty.
Assertion is a typical bahaviour of those who know what they want.
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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