C1  n. (pl. Cs or c's) 1 (also c) third letter of the alphabet. 2 mus. First note of the diatonic scale of c major. 3 third hypothetical person or example. 4 third highest category etc. 5 algebra (usu. C) third known quantity. 6 (as a roman numeral) 100. 7 (also ©) copyright.