AFFAIR - [noun] sexual relationship between people not married to each other; a set of events closely related to one another
Examples of use
She decided to divorce him after she found out that her husband has an affair with her friend.
The meeting was a noisy affair. You could hear people talking about it everywhere.
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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