Wiktionary
n. A fundamental rule, upon which other matters hinge.
Usage examples of "cardinal rule".
At midnight, when the servants again returned without his father, he sacrificed a cardinal rule of his import-export.
When Goldie Morran came to him with her plan to rid the station of Skeeter Jackson, he saw a golden opportunity to rid it of Goldie Morran, as well--a woman he knew in his bones broke the cardinal rule of time touring with every gate that opened, but was slick enough not to get caught.
Telling the truth, and nothing but the truth, is the cardinal rule.
Sure, it used to be good, but there's another cardinal rule in TV-land, and it is that nothing lasts forever.
A cardinal rule of being a successful private investigator is: Don't slug your own clients.
It was a cardinal rule to stay out of trouble with the magistrates.
He violated the cardinal rule and cautiously, examining the cost of it carefully, took something other than the operator's truth as valid.
And What about that cardinal rule of war, the concentration of force?