Chaldea was a vast plain formed by the deposits of the Euphrates and the Tigris, extending to about 400 miles along the course of these rivers, and about 100 miles in average width.
Recent discoveries, more especially in Babylonia, have thrown much light on the history of the Hebrew patriarchs, and have illustrated or confirmed the Biblical narrative in many points. The Biblical ancestor of the Hebrew people, Abraham, was born at "Ur of the Chaldees." Chaldees is a mistranslation of the Hebrew Kasdim , Kasdim being the Old Testament name of the Babylonians, while the Chaldees were a tribe who lived on the shores of the Persian Gulf, and did not become a part of the Babylonian population till the time of Hezekiah. Ur was one of the oldest and most famous of the Babylonian cities. Its site is now called Mugheir on the western bank of the Euphrates, in Southern Babylonia. About a century before the birth of Abraham it was ruled by a powerful dynasty of kings. Their conquests extended to Elam on the one side, and to the Lebanon on the other. They were followed by a dynasty of princes whose capital was Babylon, and who seem to have been of South Arabian origin. The founder of the dynasty was Sumu-abi ("Shem is my father"). But soon afterwards Babylonia fell under Elamite dominion. The kings of Babylon were compelled to acknowledge the supremacy of Elam, and a rival kingdom to that of Babylon, and governed by Elamites, sprang up at Larsa, not far from Ur, but on the opposite bank of the river. In the time of Abraham the king of Larsa was Eri-Aku, the son of an Elamite prince, and Eri-Aku, as has long been recognized, is the Biblical "Arioch king of Ellasar" (Genesis 14:1). The contemporaneous king of Babylon in the north, in the country termed Shinar in Scripture, was Khammurabi. (See Amraphel)
While the Bible claims that Abraham was from Ur of the Chaldees, a look at the time period in the book of Genesis makes it clear that if Abraham were from Ur, it would not have been yet known as the Chaldees. Very few scholars would argue that Abraham (if he existed at all) was from Ur, and therefore probably a Sumerian.
Roman and later authors used the name Chaldeans in particular for astrologers and mathematicians from Babylonia.
Initial text from Easton's Bible Dictionary, 1897. Please update as needed.