Cardinal is an adjective used to describe things—usually abstract things such as rules or principles—that are of basic or main importance. The word is also used, especially in the phrase “cardinal sin,” with the meaning “very serious or grave.”

// “Seek out multiple sources” is a cardinal rule of good news reporting.

// The four cardinal points on a compass are North, South, East, and West.

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“The cardinal rule of stargazing is going somewhere dark—the darker the skies, the better the view.” — Stefanie Waldek, Travel + Leisure, 11 Aug. 2023

Mathematics, religion, ornithology—everything seems to hinge on cardinal. As a noun, cardinal has important uses in all three of the aforementioned realms of human inquiry; as an adjective cardinal describes things of basic or main importance, suggesting that outcomes turn or depend on them. Both adjective and noun trace back to the Latin adjective cardinalis, meaning “serving as a hinge,” and further to the noun cardo, meaning “hinge.” Since the 12th century, cardinal has been used as a noun referring to a fundamentally important clergyman of the Roman Catholic Church, ranking only below the pope. (The clergyman's red robes gave the familiar North American songbird its name.) By the 1300s cardinal was also being used as the adjective we know today, to describe abstract things such as principles or rules (as opposed to, say, red wheelbarrows) upon which so much depends.