Smith's Bible Dictionary

1. Darius, the Mede, Dan_6:1; Dan_11:1, "the son of Ahasuerus," Dan_9:1, who succeeded to the Babylonian kingdom, on the death of Belshazzar, being then sixty-two years old. Dan_5:31; Dan_9:1. (B.C. 538). Only one year of his reign is mentioned, Dan_9:1; Dan_11:1, but that was of great importance for the Jews. Daniel was advanced, by the king, to the highest dignity, Dan_6:1, ff., and in his reign, was cast into the lions' den. Daniel 6. This Darius is probably the same as "Astyages," the last king of the Medes.

2. Darius, the son of Hystaspes, the founder of the Perso-Arian dynasty. Upon the usurpation of the magian Smerdis, he conspired with six other Persian chiefs to overthrow the impostor and, on the success of the plot, was placed upon the throne, B.C. 521. With regard to the Jews, Darius Hystaspes pursued the same policy as Cyrus, and restored to them, the privileges which they had lost. Ezr_5:1; Ezr_6:1; etc.

3. Darius, the Persian, Neh_12:22, may be identified with Darius II. Nothus (Ochus), king of Persia, B.C. 424-3 to 405-4; but it is not improbable that it points to Darius III. Codomannus, the antagonist of Alexander, and the last king of Persia, B.C. 336-330.

Taken from: Smith's Bible Dictionary by Dr. William Smith (1884)