The Meaning of 'Only-begotten'
The IVP Bible Background Commentary: New Testament
The tenses of the Greek verbs indicate the sense "This is how God loved the world: he gave his son."
"Only begotten" is literally "special, beloved," and was often applied in Jewish literature to Isaac, to emphasize the greatness of Abraham's sacrifice in offering him up.
The IVP Bible Background Commentary: New Testament (Downers Grove, IL.: InterVarsity Press, 1993), pp. 270-271