Definition 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

"A cardinal rule."

adjective: being or denoting a numerical quantity but not order

"Cardinal numbers."