ca.

acronym: circa
Found on http://cdiac.ornl.gov/pns/acronyms.html#C

ca.

circa; c., ca. About, around. Used in giving approximate dates, e.g. 'It happened ca. A.D. 1500.'
Found on http://www.wordinfo.info/words/index/info/view_unit/3465/3

ca.

Type: Abbreviation Definitions: 1. Abbreviation for L. circa (about, approximately).
Found on http://www.medilexicon.com/medicaldictionary.php?t=13159

Ca.

Abbreviation for circa which means approximately.
Found on http://www.encyclo.co.uk/local/22034
No exact match found