C

n. (also c) (pl. Cs or C's) 1 the third letter of the alphabet. 2 Mus. the first note of the diatonic scale of C major (the major scale having no sharps or flats). 3 the third hypothetical person or example. 4 the third highest class or category (of academic marks etc.). 5 Algebra (usu. c) the third known quantity. 6 (as a Roman numeral) 100. 7 (c) the speed of light in a vacuum. 8 (also °) copyright.