ILA’ ايلاء
A form of divorce in which a man makes a vow that he will not have connection with his wife for not less than four months and observes it inviolate. The divorce is thereby effect ipso facto, without a decree of separation from the judge. See Qur’an, Suratu ‘l-Baqarah, ii. 226: “Those who swear off from their woman, the must wait four months; but is they break their vow, God is forgiving and merciful.”
Sulaiman ibn Yasar says: “I was in company with about ten of the Prophet’s Companions, and every one said, “A man who swears that he will not go near his wife for four months shall be imprisoned until he return to her, or he shall divorce her.’” (Mishkat, book xiii. ch. xiii.)

Based on Hughes, Dictionary of Islam