TARTAN (tar'tăn, Heb. tartān). In the KJV a commander-in-chief of the Assyrian army (2Kgs.18.17; Isa.20.1). A title, not a proper name.
TARTAN tär’ tăn (תַּרְתָּן; Akkad. turtanu). Title of an Assyrian general in command of a military force and listed in the Assyrian Eponym texts as the next highest official after the king. Such officials are known from the times of Adad-nirari II, Shalmaneser III, Tiglath-pileser III, Sargon II, and Sennacherib.
Two are mentioned in the OT. The first was sent by Sargon II to capture Ashdod (Isa 20:1); the second was sent by Sennacherib with other officials (Rabsaris, Rabshakeh) to demand surrender of Jerusalem (2 Kings 18:17).
International Standard Bible Encyclopedia (1915)
For a long time the word was interpreted as a proper name, but the Assyrian inscriptions have shown it to be the title of a high official. From the eponym lists it would seem that it was the title of the highest official next to the king, which in a military empire like Assyria would be the "commander-in-chief." The Assyrian form of the name is tartanu or turtanu. In both Isa 20:1 it is used of the officer sent by Sargon, king of Assyria, against Ashdod; according to 2Ki 18:17, Sennacherib sent Tartan and RAB-SARIS (which see) and RABSHAKEH (which see) with a great host against Jerusalem. The names of the-two officials are not known.