an official order handed down by a king or governing authority: a decree, command, or ruling that those under it must obey.
This is the ordinary working word for royal authority in the parts of Ezra and Daniel written in Aramaic, the everyday language of the Persian empire's paperwork. It names the binding word that comes down from someone in power, whether it authorizes the rebuilding of the temple, forbids bowing to a statue, or threatens punishment on anyone who disobeys.
Because it was a term of the imperial bureaucracy, it settled into a few fixed phrases. The commonest by far is "to make a decree," repeated again and again as kings issue their orders. A closely tied phrase means "by decree of" or "by the authority of" someone, naming the power behind an action. It even hardens into a job title: the master of the order, that is, the commanding officer or chancellor of a province.
The same idea shows up from the other side as well: to pay heed or give proper regard to a ruler and his orders. When Daniel's accusers say his friends have paid the king no regard, they mean the men have not treated his command as binding on them.