Here is a brief introduction to the subject.
3439 monogenḗs (from 3411 /misthōtós, "one-and-only" and 1085 /génos, "offspring, stock") – properly, one-and-only; "one of a kind" – literally, "one (monos) of a class, genos" (the only of its kind).
Thayer's Greek Lexicon
STRONGS NT 3439...