It appears that the Chaldeans (Kaldai or Kaldi) were in the earliest times merely one out of many Cushite tribes inhabiting the great alluvial plain known afterwards as Chaldea or Babylonia. Their special seat was probably that southern portion of the country which is found to have so late retained the name of Chaldea. In process of time, as the Kaldi grew in power, their name gradually prevailed over those of the other tribes inhabiting the country; and by the era of the Jewish captivity it had begun to be used generally for all the inhabitants of Babylonia. It appears that while, both in Assyria and in later Babylonia, the Shemitic type of speech prevailed for civil purposes, the ancient Cushite dialect was retained, as a learned language for scientific and religious literature. This is no doubt the "learning" and the "tongue" to which reference it made in the book of Daniel, (Daniel 1:4) The Chaldeans were really the learned class; they were priests, magicians or astronomers, and in the last of the three capacities they probably effected discoveries of great importance. In later times they seem to have degenerated into mere fortune-tellers.
Chaldeans
The word Chaldeans carries two meanings in these verses. In Jeremiah it names a nation: “A sword is upon the Chaldeans, saith the LORD, and upon the inhabitants of Babylon, and upon her princes, and upon her wise men.” In Daniel it also names a group at the king’s court, listed beside “the magicians, and the astrologers, and the sorcerers.” Daniel and his friends are taught “the learning and the tongue of the Chaldeans”, yet when the king needs a dream explained, these experts cannot make it known. Daniel 2 is the place to see that scene unfold.
5 verses
Jeremiah
A sword is upon the Chaldeans, saith the LORD, and upon the inhabitants of Babylon, and upon her princes, and upon her wise men.
Daniel
Children in whom was no blemish, but well favoured, and skilful in all wisdom, and cunning in knowledge, and understanding science, and such as had ability in them to stand in the king's palace, and whom they might teach the learning and the tongue of the Chaldeans.
Then the king commanded to call the magicians, and the astrologers, and the sorcerers, and the Chaldeans, for to shew the king his dreams. So they came and stood before the king.
Then came in the magicians, the astrologers, the Chaldeans, and the soothsayers: and I told the dream before them; but they did not make known unto me the interpretation thereof.
The king cried aloud to bring in the astrologers, the Chaldeans, and the soothsayers. And the king spake, and said to the wise men of Babylon, Whosoever shall read this writing, and shew me the interpretation thereof, shall be clothed with scarlet, and have a chain of gold about his neck, and shall be the third ruler in the kingdom.
From Smith's Bible Dictionary
William Smith, 1884