"So what does monogenes mean? According to the Greek-English Lexicon of the New Testament and Other Early Christian Literature (BAGD, 3rd Edition), monogenes has two primary definitions. The first definition is 'pertaining to being the only one of its kind within a specific relationship.' This is its meaning in Hebrews 11:17 [where] the writer refers to Isaac as Abraham's 'only begotten son' (KJV). Abraham had more than one son, but Isaac was the only son he had by Sarah and the only son of the covenant. Therefore, it is the uniqueness of Isaac among the other sons that allows for the use of monogenes in that context." (Note: my emphasis.)
"The second definition is 'pertaining to being the only one of its kind or class, unique in kind.' This is the meaning that is implied in John 3:16 (see also John 1:14, 18; 3:18; 1 John 4:9). John was primarily concerned with demonstrating that Jesus is the Son of God (John 20:31), and he uses monogenes to highlight Jesus as uniquely God's Son—sharing the same divine nature as God—as opposed to believers who are God's sons and daughters by adoption (Ephesians 1:5). Jesus is God’s 'one and only' Son." (Note: my emphasis.)
[We believe in ] . . . in ONE Lord Jesus Christ, (Acts 11:17)
the Son of God, (Mathew 14:33; 16:16)
the Only-Begotten, (John 1:18; 3:16)
Begotten of the Father before all ages. (John 1:2)
Light of Light; (Psalm 27:1; John 8:12; Matthew 17:2,5)
True God of True God; (John 17:1-5)
Begotten, not made; (John 1:18)
of one essence with the Father (John 10:30)
by whom all things were made; (Hebrews 1:1-2)
Who for us men and for our salvation (1Timothy 2:4-5)
came down from Heaven, (John 6:33,35)
and was incarnate of the Holy Spirit and the Virgin Mary, (Luke 1:35)
and became man. (John 1:14)