Definitions of cardinal:

• noun:   crested thick-billed North American finch having bright red plumage in the male
• noun:   a variable color averaging a vivid red
• noun:   (Roman Catholic Church) one of a group of more than 100 prominent bishops in the Sacred College who advise the Pope and elect new Popes
• noun:   the number of elements in a mathematical set; denotes a quantity but not the order
• adjective:   serving as an essential component

  Example: "A cardinal rule"

• adjective:   being or denoting a numerical quantity but not order

  Example: "Cardinal numbers"

• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #7622)