(1.) In the natural and common sense (Matthew 1:2; Luke 3:1, 19).

(2.) A near relation, a cousin (Genesis 13:8; 14:16; Matthew 12:46; John 7:3; Acts 1:14; Galatians 1:19).

(3.) Simply a fellow-countryman (Matthew 5:47; Acts 3:22; Hebrews 7:5).

(4.) A disciple or follower (Matthew 25:40; Hebrews 2:11, 12).

(5.) One of the same faith (Amos 1:9; Acts 9:30; 11:29; 1 Corinthians 5:11); whence the early disciples of our Lord were known to each other as brethren.

(6.) A colleague in office (Ezra 3:2; 1 Corinthians 1:1; 2 Corinthians 1:1).

(7.) A fellow-man (Genesis 9:5; 19:7; Matthew 5:22, 23, 24; 7:5; Hebrews 2:17).

(8.) One beloved or closely united with another in affection (2 Samuel 1:26; Acts 6:3; 1 Thessalonians 5:1). Brethren of Jesus (Matthew 1:25; 12:46, 50: Mark 3:31, 32; Galatians 1:19; 1 Corinthians 9:5, etc.) were probably the younger children of Joseph and Mary. Some have supposed that they may have been the children of Joseph by a former marriage, and others that they were the children of Mary, the Virgin's sister, and wife of Cleophas. The first interpretation, however, is the most natural.

Noah Webster's New International Dictionary of the English Language

1. (n.) 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.

2. (n.) 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.

3. (n.) One who, or that which, resembles another in distinctive qualities or traits of character.

4. (v. t.) To make a brother of; to call or treat as a brother; to admit to a brotherhood.