a sexual relationship between two people, usually when one or both of them are already in a relationship with somebody else
خیانت, رابطه پنهانی, عشقبازی مخفیانه, عشق پنهان, ماجرای عشقی
She was having an affair with a co-worker.
It's the story of a secret affair between a married teacher and her teenage student.
see also love affair