Jacob is a transliteration of the Hebrew יַעֲקֹב (formal transliteration: yaʿăqob). This labels only one person in the Hebrew Bible: the patriarch also known as Israel, the eponymous ancestor of the twelve tribes.
The Septuagint and the Greek New Testament transliterate this word as Ἰακώβ (Iakōb). It most often refers to the patriarch. Once (Matt 1:16), it refers to the father of Joseph the husband of Mary.
The Semitic name Ἰακώβ (Iakōb) was Hellenized to Ἰάκωβος (Iakōbos), following the common pattern of adding -ος (-os) to form a Greek male name. Ἰάκωβος is the name of:
- the son of Zebedee (Matt 4:21 et al.);
- the brother of Jesus (Matt 13:55, Acts 15:13, Gal 1:19 et al.);
- the son of Alphaeus (Matt 10:3 et al.; perhaps the same as James the younger, Mark 15:40); and
- the father of Judas (not Iscariot, Luke 6:16, Acts 1:13).1
It is not surprising that Hebrew yaʿăqob and its Hellenized form Iakōbos would move into a third language as the same name. The twist here is the English choice of "James" for the latter only. This traditional rendering is based on the Latin Iacomus, an alternative for the Vulgate NT's Iacobus.2 In contrast, the Hebrew yaʿăqob moved into Latin as simply Iacob, whence "Jacob."
1. List courtesy of: "Ἰακώβ, Ἰάκωβος" in New International Dictionary of New Testament Theology and Exegesis. Ed. Moises Silva (Zondervan, 2014), 2:491.
2. Per Wikipedia: Iacobus > *Iacombus > Iacomus via nasalization of the o and assimilation to the following b followed by simplification of the cluster mb through loss of the b. That's my story and I'm sticking to it!