brother

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A male having the same parents as another or one parent in common with another.
• noun One who shares a common ancestry, allegiance, character, or purpose with another or others, especially.
• noun A kinsman.
• noun A fellow man.
• noun A fellow member, as of a fraternity, trade union, or panel of judges on a court.
• noun A close male friend; a comrade.
• noun A fellow African-American man or boy.
• noun Something, such as a corporation or institution, that is regarded as a member of a class.
• noun A lay member of a religious order of men.
• noun A fellow member of the Christian church.

from The Century Dictionary.

• Bearing a fraternal relation in a general sense; of the character of a brother: as, a brother man or magistrate.
• To consider or treat as a brother; address as a brother.
• To relate as brothers; make kin.
• noun A male person, in his relation to another person or other persons of either sex born of the same parents; a male relative in the first degree of descent or mutual kinship: used also of the lower animals: the converse of sister. See brother-in-law and half-brother.
• noun A male person in his relation to any other person or persons of the same blood or ancestry; a member of a common family or race in his relation to all other members; in the plural, all members of a particular race, or of the human race in general, as regards each other.
• noun One of two or more men closely united without regard to personal kinship, as by a common interest; an associate; one of the same rank, profession, occupation, or belief, especially in law, religion, or organized charity.
• noun Specifically, as a translation of friar, a member of a mendicant order.
• noun In the plural form brethren, the designation of several Christian organizations, derived from the fact that the title was used by the primitive Christians in speaking of themselves; specifically, a sect of German Baptists, more popularly known as Dunkers.
• noun A member of a religious congregation whose members do not receive the priesthood, but devote themselves to teaching or good works; also, a lay member of a community having priests.
• noun Figuratively, one who resembles another in manners or disposition.
• noun Often abbreviated bro., plural bros.
• noun [The plural form brethren is not now used in the sense of male children of the same parents, but only in the wider meanings of the word brother.]

from the GNU version of the Collaborative International Dictionary of English.

• noun A male person who has the same father and mother with another person, or who has one of them only. In the latter case he is more definitely called a half brother, or brother of the half blood.
• noun One related or closely united to another by some common tie or interest, as of rank, profession, membership in a society, toil, suffering, etc.; -- used among judges, clergymen, monks, physicians, lawyers, professors of religion, etc.
• noun One who, or that which, resembles another in distinctive qualities or traits of character.
• noun a humorous designation for the people of the United States collectively. The phrase is said to have originated from Washington's referring to the patriotic Jonathan Trumbull, governor of Connecticut, as “Brother Jonathan.”
• noun See under Blood.
• transitive verb To make a brother of; to call or treat as a brother; to admit to a brotherhood.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Son of the same parents as another person.
• noun A male child descended from the same parents.
• noun A male having at least one parent in common with another (see half-brother, stepbrother).
• noun A male fellow member of a religious community, church, trades union etc.
• noun African American Vernacular A black male.
• noun Someone who is a peer, whether male or female.
• verb transitive To treat as a brother.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (Roman Catholic Church) a title given to a monk and used as form of address
• noun a close friend who accompanies his buddies in their activities
• noun a male with the same parents as someone else
• noun used as a term of address for those male persons engaged in the same movement
• noun a male person who is a fellow member (of a fraternity or religion or other group)