Brother, from the Old English word brothor and cognate with the Latin term frater and the Greek word phrater (both of which mean “fellow clan member”), means not only “a male with one or more parents in common” but has also come, by extension, to refer to a man with whom one has a bond or a common interest.
It also applies to national or racial commonality, as in the term “soul brother,” which in American English describes a black male. In addition, it can refer loosely to a male relative or generically to something that is similar to something else. In religious contexts, it denotes a minister or a member of a religious order who has not been ordained.
The plural is either brothers or, in formal and religious contexts, the archaic form brethren. The quality of being a brother, literally or figuratively, is brotherhood, and brotherly is the adverbial form.
A blood brother is literally a brother by birth or figuratively someone with whom one shares a bond of loyalty; originally, the term alluded to the ceremonial exchange of blood between two men, often by mingling blood at the point of a slight self-inflicted wound.
Brother-german is a technical legal term pertaining to the default definition of brother—“a man or boy who has both of the same parents as a given person,” as opposed to a half brother, who shares only one parent, or a stepbrother, the son of a stepparent. Likewise, a sister-german shares both parents with a given person. (The term german, from the Latin word germanus, means “having the same parents” and is unrelated to the proper noun referring to a person from Germany.)
Brother-in-arms originally strictly referred to a fellow combatant in the same military service, but by extension it alludes to anyone one is closely associated with. (Because women have only recently had a significant role in the military, no equivalent term developed for female soldiers, but the term sisters-in-arms has been employed sporadically, such as in the title of a documentary about female soldiers in combat.)
Idiomatic uses of brother include “brother’s keeper,” a reference to the biblical exchange in which Cain protests, “Am I my brother’s keeper?” when God asks the whereabouts of Cain’s brother Abel, whom Cain has killed. (The contemporary notion behind the phrase is of interdependent responsibility among people.) Meanwhile, “Big Brother” is a reference (from George Orwell’s novel Nineteen Eighty-Four) to an all-seeing authoritarian leader or any government entity that practices oppressive surveillance or control. However, “big brother” also refers generically to one’s older male sibling or to a man who mentors a boy to whom he is not related.
Recent idioms include bromance, a portmanteau word from brother and romance, pertaining to depictions in popular culture of close platonic friendships among men, and brogrammer, a mash-up of brother and programmer that alludes to assertive, masculine computer programmers, a divergence from the stereotype of technologically adept but physically and socially awkward males.