c has 21 distinct synonyms spread across 10 of its 13 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a degree on the centigrade scale of temperature
the speed at which light travels in a vacuum; the constancy and universality of the speed of light is recognized by defining it to be exactly 299,792,458 meters per second
a vitamin found in fresh fruits (especially citrus fruits) and vegetables; prevents scurvy
one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
a base found in DNA and RNA and derived from pyrimidine; pairs with guanine
an abundant nonmetallic tetravalent element occurring in three allotropic forms: amorphous carbon and graphite and diamond; occurs in all organic compounds
a unit of electrical charge equal to the amount of charge transferred by a current of 1 ampere in 1 second
a general-purpose programing language closely associated with the UNIX operating system
(music) the keynote of the scale of C major
being ten more than ninety
Synonyms, antonyms and the broader and narrower relations for c come from Princeton University’s WordNet. Full sources and licences.