In the Greek version, the Septuagint, the Hebrew bin’cha yechid’cha is rendered “your beloved son”. The Jewish Publication Society of America version says “your favoured one”.
The rabbinic interpretations all accept the literal meaning of yachid as “only one”.
But Abraham had two sons, Ishmael and Isaac!
Hence Rashi explains that Isaac was the only son of his mother, Sarah, whilst Ishmael was the only son of his mother, Hagar.
Maimonides and Abravanel say that because Ishmael was banished, it was as if Isaac were the only son.
Nachmanides says that Isaac was the only son of the main wife, Sarah.
Tiferet Yisrael argues, a little pedantically, that Isaac was the only son of Abraham, whose name had been Abram when Ishmael was born.
The view of the Shach is that Isaac was the only son of any moral quality.