Definition of Fay’
Linguistically, the Arabic word Fay’ means returning and resuming. The Arabs would use it to describe the case when the husband takes an oath to stay away from his wife but then returns to her.  However, in Islam, the technical term refers to the case when the husband resumes having intercourse with his wife after he has taken an oath not to have sex with her.

2. Requirements of Fay’
a. Resuming the married life by action: The scholars are in consensus that the husband meets the requirement of Fay’ once he has intercourse with his wife before the end of the period of Eelaa’. The consensus is reported by Ibn ‘Abd al-Barr, Ibn al-Munthir, and al-Nawawee.
b. Resuming the married life by words: The four Madhabs agree that Fay’s can be established by words only if the husband is unable to establish it by action.