A particularly interesting development in the history of Christianity [related to translation] took place with respect to the Greek term monogenés, literally, ‘only, unique, one of kind.’ It was used of Isaac as the son of Abraham [see Gen. 22:2], though Isaac was not the only son of Abraham, for he had a son Ishmael, and with a later wife Keturah, several sons. But Isaac was the only son of a particular kind, that is to say, the unique son of the promise. The term monogenés was translated into Latin as unigenitus, meaning literally ‘only begotten’ [in English — or likewise traditionally in Chinese: “dúshēng 獨 生,” Italian: “unigenito,” Spanish “unigénito,” or German: “eingeboren”] but in Greek the equivalent of ‘only begotten’ would have two n’s and not just one. Nevertheless, the Latin misinterpretation of monogenés has constituted such a long tradition that any attempt to speak of Jesus as the ‘unique son of God’ rather than the ‘only begotten son’ is often announced as a case of blatant heresy.
Source: Nida 1984, p. 114.
See also complete verse (John 3:16).