Webster's Revised Unabridged Dictionary · 1913

Brother

Brother (brut"ẽr) , noun

[Old English brother, Anglo-Saxon brōeor; akin to Old Saxon brothar, Dutch broeder, Old High German pruodar, German bruder, Icelandic brōeir, Swedish & Danish broder, Gothic brōþar, Ir. brathair, Welsh brawd, pl. brodyr, Lithuanian brolis, Lett. brahlis, Russ. brat', Pol. & Serv. brat, OSlav. bratru, Latin frater, Sanskrit bhrātr, Zend bratar brother, Greek fra`thr, fra`twr, a clansman. The common plural is Brothers; in the solemn style, Brethren, Old English pl. brether, bretheren, Anglo-Saxon dative sing. brēeer, nom. pl. brōeor, brōeru. r258. Compare Friar, Fraternal.]

A brother having the same mother but different fathers is called a uterine brother, and one having the same father but a different mother is called an agnate brother, or in (Law) a consanguine brother. A brother having the same father and mother is called a brother-german or full brother. The same modifying terms are applied to sister or sibling.

1.
(Law) 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.
Two of us in the churchyard lie, My sister and my brother. — Wordsworth
2.
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.
A brother of your order. — Shakespeare
We few, we happy few, we band of brothers, For he to-day that sheds his blood with me Shall be my brother. — Shakespeare
3.
One who, or that which, resembles another in distinctive qualities or traits of character.
He also that is slothful in his work is brother to him that is a great waster. — Bible (KJV) - Proverb xviii. 9
That April morn Of this the very brother. — Wordsworth
For of whom such massacre Make they but of their brethren, men of men? — Milton

In Scripture, the term brother is applied to a kinsman by blood more remote than a son of the same parents, as in the case of Abraham and Lot, Jacob and Laban. In a more general sense, brother or brethren is used for fellow-man or fellow-men.

Collocations (2)
Brother Jonathan , 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.”
Blood brother , See under Blood.

Brother ({not transcribed}) , transitive verb

To make a brother of; to call or treat as a brother; to admit to a brotherhood. — Sir W. Scott