ZIHAR
Muhammad Asad explains this term in his commentary on Sura 33:4 in this way:
This is a reference to the pre-Islamic Arabian custom called zihar, whereby a husband could divorce his wife by simply declaring, “Thou art [henceforth as unlawful] to me as my mother’s back”, the term zahr (“back”) being in this case a metonym for “body”. In pagan Arab society, this mode of divorce was considered final and irrevocable; but a woman thus divorced was not allowed to remarry, and had to remain forever in her former husband’s custody. ... (p. 639, fn. 3)
The specific injustice of this divorce is that the woman virtually becomes a household slave. She looses her rights and status as a spouse but is not free to marry anyone else. Muhammad abolished this kind of divorce, or at least its finality, in Sura 58:1-4.
Hughes' Dictionary of Islam provides this explanation:
ZIHAR. Lit. "Likening to the back." A form of impreccation which involves the separation of husband and wife until expiation is made. According to the Hidaya, zihar signifies the likening of a woman to a kinswoman within the prohibited degrees, which interpretation is found in the comparison being applied to any of the parts or members of the body improper to be seen. The usual formula is: Anti ‘alaiya ka-zahri ummi, "Thou art unto me as my mother's back."
Before the establishment of Muhammadanism, zihar stood as a divorce, but Muhammad changed it to a temporary prohibition for which expiation must be performed, viz. either freeing a slave, or two months' fast, or feeding sixty persons. ...
Wherry gives the historical background to this revelation in his commentary on S. 58:1:
Her who disputed. "This was Khaula Bint Thalaba, the wife of Aus Ibn al Samat, who being divorced by her husband by a form in use among the Arabs in the time of ignorance, viz., by saying to her, 'Thou art to me as the back of my mother,' came to ask Muhammad's opinion whether they were necessarily obliged to a separation; and he told her that it was not lawful for her to cohabit with her husband any more: to which she replying that her husband had not put her away, the Prophet repeated his former decision, adding that such form of speaking was, by general consent, understood to imply a perpetual separation. Upon this the woman, being greatly concerned because of the smallness of her children, went home and uttered her complaint to God in prayer: and thereupon this passage was revealed, allowing a man to take his wife again, notwithstanding his having pronounced the above-mentioned form of divorce, on doing certain acts of charity or mortification by way of penance. (E. M. Wherry, A Comprehensive Commentary on the Qur'an, Vol. 4, p. 124)