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 the dress code practice of hijab.

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 isn't a general consensus that the word should be capitalized like Muslim. (The Arabic alphabet of course has no upper-case vs. lower-case distinction.)

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

Who is mahram?

Anyone whom 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 Quran.

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

1. permanent or blood mahrams with whom one is mahram by a blood relationship:
   1. father, grandfather, great-grandfather and so on;
   2. brother;
   3. son, grandson, great-grandson;
   4. uncle, parents' uncle, grandparents' uncle and so on;
   5. nephew, grandnephew, great-grandnephew and so on;
2. in-law mahrams with whom one becomes mahram by marrying someone:
   1. father-in-law;
   2. son-in-law;
   3. stepfather (mother's husband) if their marriage is consummated;
   4. stepson (husband's son) if her marriage is consummated;
3. rada or milk-suckling mahrams with whom you become mahram because of being nursed by a woman. When a woman acts as a wetnurse (that is, she breast feeds an infant that is not her own child) for a certain amount of time under certain conditions, she becomes the child's rada mother and all said about blood mahrams applies here, like rada father/mother, rada sister/brother, rada aunt/uncle and so on. In English these can be referred to as milk brother, milk-mother, etc. (See also breastfeeding fatwa.)
4. All non-Muslim Males as mahrams:
   1. All non-Muslim men are "mahram" (forbidden) for a Muslim woman to marry. The Quran specifically forbids a Muslim woman from marrying any non-Muslim man, which is today still recognized as applicable both by Muslim and non-Muslim experts alike.^[1]

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Some rules regarding mahrams and ghayr mahrams (non-mahrams)

• Theoretically, a Muslim woman's mahrams form the group of allowable escorts when she travels.
• An adopted brother (adopted sister) of a woman (man) is ghayr mahram to her (him) and they can marry each other. The term "adopted" means those children who are adopted by one's parents for the purpose of providing shelter and upbringing and who do not fall under the relationships outlined under the section "Who is Mahram?" above.
• Except for the spouse, being mahram is a permanent condition. That means, for example, a man will remain mahram to his ex-mother-in-law after divorcing her daughter. One is ghayr mahram to one's ex-spouse.
• One must not stay with a ghayr mahram in seclusion where none of their mahrams is present (see also proxemics).
• A rada sister (brother) is only mahram to the boy (girl) that her (his) mother has breast fed. That means she (he) is ghayr mahram to his (her) other brothers (sisters) unless the mother has fed them separately.
• If wives of a man each become a rada mother of a child, all children and all rada mothers will be mahram to each other.

http://www.usc.edu/schools/college/crcc/engagement/resources/texts/muslim/quran/002.qmt.html#002.221