Mahram

In Islamic sharia legal terminology, a mahram (Arabic محرم, also transliterated mahrim or maharem) is an unmarriageable kin with whom sexual intercourse would be considered incestuous, a punishable taboo. Current usage of the term covers a wider range of people and mostly deals with hijab and its cultural and regional norms.

The plural form of the word in the Arabic language is maharim with long second vowel (Arabic محارم, also transliterated maharem). Sometimes the word is capitalized but there is not a general consensus that the word should be capitalized like Muslim. (The Arabic alphabet has no upper-case vs. lower-case distinction.)

Being mahram is a mutual condition. If A is mahram to B, B is definitely mahram to A.

Any relative that a Muslim is not allowed to marry is mahram, if they are of the opposite sex and have reached puberty. A partial list of what is considered a mahram can be found in Surah 24, Ayah 31, of the Koran.

A woman's male mahrams fall into four categories (three categories in the strict-sense definition that does not count one's spouse). Mahrams for a man can be derived in a similar manner.

For a man, mahram women include his mother, grandmother, daughter, granddaughter, sister, aunt, grandaunt, niece, grandniece, his father's wife, his wife's daughter (step-daughter), his mother-in-law, his rada mother and any other rada relatives that correspond to the above-mentioned blood relatives. As the Prophet said, "What is forbidden by reason of kinship is forbidden by reason of suckling." (Al-Bukhari)

These are considered maharim because they are mentioned in the Koran:

"And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way. Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your foster mother who gave you suck, your foster milk suckling sisters, your wives' mothers, your step-daughters under your guardianship, born of your wives to whom you have go in - but there is no sin on you if you have not gone in them (to marry their daughters), ... the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allah is Oft-Forgiving, Most Merciful." (An-Nisa 4:22-23)

...
Wikipedia