The word targum (plural targumim or, in English, targums), meaning “translation,” can be applied to any translation, but is used specifically to designate the Aramaic version of the Bible.
The Targumim have their origin at about the turn of the eras BCE-CE, a time when Hebrew was becoming less widely understood than Aramaic among Jewish congregations. In synagogue worship the practice arose of giving an oral translation into Aramaic, following the reading of each verse of the Torah and also the Haphtarah, the set reading from the Prophets, though it is possible that the activity of translation began at an earlier stage in the school and academy to meet an educational need. These translations were not always literal, but at times brought in aggada (traditional stories based on the biblical text) and midrashic material, and might have more of the character of paraphrase than translation.
To the Pentateuch
Targum was at first a purely oral activity, but it was not long before written targumim began to emerge. Targum Onqelos soon became widely accepted as the most authoritative Targum to the Pentateuch, and enjoyed an official status. It is likely that Targum Onqelos originated in Palestine in the 1st or 2nd century CE, and was then taken to Babylon, where it underwent a thorough revision in about the 4th-5th centuries CE, before returning to Palestine at a later stage.
The other Targumim to the Pentateuch represent a more characteristically Palestinian tradition. Targum Pseudo-Jonathan is otherwise known as Targum Yerushalmi I. Its present form probably does not date before the 7th century CE.
The Fragment Targum, or Targum Yerushalmi II, probably represents an earlier Palestinian Targum than Pseudo-Jonathan. As the name suggests, this is an incomplete Targum, covering only certain passages of the Pentateuch.
An important version of the Palestinian Targum to the whole of the Pentateuch is be found in Codex Neofiti I of the Vatican Library. It is probably to be dated not later than 3rd-4th centuries CE.
To the Prophets
The Targum to the Former Prophets (Joshua to Kings) and Latter Prophets (Isaiah to Malachi) is known as Targum Jonathan. It probably originated in Palestine at about the same time as Onqelos, and then underwent revision in Babylonia, where it was regarded as authoritative and became official.
To the Writings
There are also Targumim to all the books of the Writings, with the exception of Daniel and Ezra-Nehemiah (both of which have some passages in Aramaic). These Targumim never enjoyed the same authoritative status as Onqelos and Jonathan, and probably all originated at a relatively late date.
Alexander, Philip S. “Targum, Targumim.” Pages 320b-331b in vol. 6 of The Anchor Bible Dictionary. 6 vols. Edited by David Noel Freedman. New York: Doubleday, 1992.
McNamara, Martin. Targum and Testament Revisited: Aramaic Paraphrases of the Hebrew Bible. 2nd ed. Grand Rapids, MI: Eerdmans, 2010.