Jamieson, Fausset and Brown on Daniel 5:10

10. queen--the queen mother, or grandmother, Nitocris, had not been present till now. She was wife either of Nebuchadnezzar or of Evil merodach; hence her acquaintance with the services of Daniel. She completed the great works which the former had begun. Hence Herodotus attributes them to her alone. This accounts for the deference paid to her by Belshazzar. (See on Da 4:36). Compare similar rank given to the queen mother among the Hebrews (1Ki 15:13).