Law of the Medes and Persians

Etymology
From the King James Bible, Daniel Chapter 6, verse 8:
 * "Now, O king, establish the decree, and sign the writing, that it be not changed, according to the law of the Medes and Persians, which altereth not."

Noun

 * 1) A rule, law or custom which is unchangeable.

Translations

 * Greek: Νόμος των Μήδων και των Περσών