ἀδελφός, -οῦ, ὁ (from α copulative and δελφύς, from the same womb; cf. ἀγάστωρ) [from Homer down];
1. a brother (whether born of the same two parents, or only of the same father or the same mother): Matthew 1:2; Matthew 4:18, and often. That 'the brethren of Jesus,' Matthew 12:46, 47 [but WH only in marginal reading]; Matthew 13:55f; Mark 6:3 (in the last two passages also sisters); Luke 8:19; John 2:12; John 7:3; Acts 1:14; Galatians 1:19; 1 Corinthians 9:5, are neither sons of Joseph by a wife married before Mary (which is the account in the Apocryphal Gospels [cf. Thilo, Cod. Apocr. N. T. i. 362f]), nor cousins, the children of Alphæus or Cleophas [i. e. Clopas] and Mary a sister of the mother of Jesus (the current opinion among the doctors of the church since Jerome and Augustine [cf. Bp. Lightfoot's Commentary on Galatians, diss. ii.]), according to that use of language by which ἀδελφός like the Hebrew אָח denotes any blood-relation or kinsman (Genesis 14:16; 1 Samuel 20:29; 2 Kings 10:13; 1 Chronicles 23:22, etc.), but own brothers, born after Jesus, is clear principally from Matthew 1:25 [only in R G]; Luke 2:7 — where, had Mary borne no other children after Jesus, instead of υἱὸν πρωτότοκον, the expression υἱὸν μονογενῆ would have been used, as well as from Acts 1:14, cf. John 7:5, where the Lord's brethren are distinguished from the apostles.
See further on this point under Ἰάκωβος, 3. [Cf. B. D. under the word Brother; Andrews, Life of our Lord, pp. 104-116; Bib. Sacr. for 1864, pp. 855-869; for 1869, pp. 745-758; Laurent, N. T. Studien, pp. 153-193; McClellan, note on Matthew 13:55.]
2. according to a Hebrew use of אָח (Exodus 2:11; Exodus 4:18, etc.), hardly to be met with in secular authors, having the same national ancestor, belonging to the same people, countryman; so the Jews (as the σπέρμα Ἀβραάμ, υἱοἰ Ἰσραήλ, cf. Acts 13:26; [in Deuteronomy 15:3 opposed to ὁ ἀλλότριος, cf. Acts 17:15; Acts 15:12; Philo de septen. § 9 at the beginning]) are called ἀδελφοί: Matthew 5:47; Acts 3:22 (Deuteronomy 18:15); Acts 7:23; 22:5; 28:15, 21; Romans 9:3; in address, Acts 2:29; Acts 3:17; Acts 23:1; Hebrews 7:5.
3. just as in Leviticus 19:17 the word אָח is used interchangeably with רֵַעַ (but, as Leviticus 19:16, 18 show, in speaking of Israelites), so in the sayings of Christ, Matthew 5:22, 24; Matthew 7:3ff, ἀδελφός is used for ὁ πλησίον to denote (as appears from Luke 10:29ff) any fellow-man — as having one and the same father with others, viz. God (Hebrews 2:11), and as descended from the same first ancestor (Acts 17:26); cf. Epictetus diss. 1, 13, 3.
4. a fellow-believer, united to another by the bond of affection; so most frequently of Christians, constituting as it were but a single family: Matthew 23:8; John 21:23; Acts 6:3 [Lachmann omits]; Acts 9:30; 11:1; Galatians 1:2; 1 Corinthians 5:11; Philippians 1:14, etc.; in courteous address, Romans 1:13; Romans 7:1; 1 Corinthians 1:10; 1 John 2:7 Rec., and often elsewhere; yet in the phraseology of John it has reference to the new life unto which men are begotten again by the efficiency of a common father, even God: 1 John 2:9ff; 1 John 3:10, 14; etc., cf. 1 John 5:1.
5. an associate in employment or office: 1 Corinthians 1:1; 2 Corinthians 1:1; 2 Corinthians 2:13 (12); Ephesians 6:21; Colossians 1:1.
6. brethren of Christ is used of,
a. his brothers by blood; see 1 above.
b. all men: Matthew 25:40 [Lachmann brackets]; Hebrews 2:11f [others refer these examples to d.]
c. apostles: Matthew 28:10; John 20:17.
d. Christians, as those who are destined to be exalted to the same heavenly δόξα (which see, III. 4 b.) which he enjoys: Romans 8:29.

BLB Scripture Index of Thayer's