one and only: a parent's single or uniquely irreplaceable child, and so of Jesus the only Son of God.
The word joins "one, only" with a root for family or kind, so at its heart it points to a child who stands alone: the single one a parent has. In everyday Greek and in Luke it carries the ache of an irreplaceable child, the widow's only son, a father's dying daughter.
John lifts the same word to describe Jesus as the one and only Son of God, unlike anyone else. Whether it also carries the idea of being "begotten" is one of the most argued questions in New Testament study. The word has only one "n" where a form built from the verb "to beget" would have two, and most scholars now read it simply as "the only Son," one of a kind, though a serious minority hears a note of begetting in family settings.
Hebrews stretches the word further still. It calls Isaac Abraham's only son even though Abraham had other children, because Isaac alone was the son God had promised. So the word can mean uniquely the son, not merely the sole offspring.
Across all its uses one thread holds: this child is the only one of his or her kind to a parent. What changes is the setting, from the plain grief of a Galilean home, to the singular place of Isaac in God's plan, to the unrepeatable Son who makes God known.
Nuances
Whether the original text of John 1:18 read "only God" or "only Son," and whether the "God" reading is original or a later scribal heightening of the text.
- majority"Only God" is original: it has the earliest manuscript support and is the harder reading that scribes would naturally smooth toward the more familiar "only Son."; Bruce M. Metzger, A Textual Commentary on the Greek New Testament, 2nd ed. (Stuttgart, 1994), 169; Murray J. Harris, Jesus as God: The New Testament Use of Theos in Reference to Jesus (Grand Rapids, 1992), 74-103
- minority"Only Son" is original: it matches John's ordinary wording in 3:16, 3:18, and 1 John 4:9, whereas "only God" is an abrupt and awkward phrase.; C. K. Barrett, The Gospel According to St John, 2nd ed. (London, 1978), 169; Rudolf Schnackenburg, The Gospel According to St John, vol. 1 (ET New York, 1968), 279-280
- minority"Only God" is a later orthodox alteration, introduced to strengthen the claim that Jesus is God against lower views of Christ.; Bart D. Ehrman, The Orthodox Corruption of Scripture, rev. ed. (Oxford, 2011), 78-82