What is a…
brother

This word has different meanings in the Bible:

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; Heb. 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

   See: Genesis 9:5; 19:7; Matthew 5:22-24; 7:5; Hebrews 2:17

8. one beloved or closely united with another in affection

   See: 2 Samuel 1:26; Acts 6:3; 1 Thess. 5:1

The brothers 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.