#1 cardinal s. [central, fundamental, key, primal] serving as an essential component; "a cardinal rule"; "the central cause of the problem"; "an example that was fundamental to the argument"; "computers are fundamental to modern industrial structure". wn
#2 cardinal n. (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. wn
#3 cardinal n. [cardinal number] the number of elements in a mathematical set; denotes a quantity but not the order. wn
#4 cardinal a. being or denoting a numerical quantity but not order; "cardinal numbers". wn
#5 cardinal n. [carmine] a variable color averaging a vivid red. wn
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