The Collaborative International Dictionary of English v.0.48:

Ca \Ca\ n.
   the chemical symbol for calcium, the fifth most abundant
   element in the earth's crust, having an atomic number of 20.

   Syn: calcium.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

circa \cir"ca\ (s[~e]r"k[.a]), adv., prep. [L.]
   Approximately; about; commonly abbreviated ca.; -- used
   especially before dates and numerical measures; as, he was
   born circa 1650; ca. 50 feet high.

   Syn: ca.
        [PJC]