What is meant by "only begotten son"?
The phrase is used several times by John:
John 1:14 And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.
John 1:18 No one has seen God at any time. The only begotten Son, who is in the bosom of the Father, He has declared Him.
John 3:16 For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.
John 3:18 He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.
1 John 4:9 In this the love of God was manifested toward us, that God has sent His only begotten Son into the world, that we might live through Him.
1 John 5:1 Whoever believes that Jesus is the Christ is born of God, and everyone who loves Him who begot also loves him who is begotten of Him.
The normal sense of the word means to father a son. As in the case of Adam and Abraham: Genesis 5:4; Hebrews 11:17
Jesus came into existence by the fertilisation of the egg inside Mary by the power of God. He was the only man to come into existence in this way. Luke 1:35
In a 'spiritual' sense, Jesus was begotten again when God raised him from the dead and gave him the divine nature. This was prophesied in Psalm 2:7; Acts 13:33; Hebrews 1:5; Hebrews 5:5; Revelation 1:5.
The phrase, "only begotten son", excludes the idea that Jesus pre-existed before he was born.
I hope you find this helpful.
God bless,
Glenn