11th commandment

Etymology
In reference to the biblical Ten Commandments, carrying the implication that the convention is of very high importance (similar to the Ten Commandments).

Noun

 * 1)  A well-known convention (often the most well-known of a certain field) which supposedly can not or should not be broken.

Usage notes
The "commandment" to which is referred often includes "thou shalt not," or similar phrases, in imitation of the biblical Commandments.