Circa is Latin for "around" or "about". It is often used to show when something approximately happened. It is often shortened to c., ca., ca or cca.
An encyclopedia entry may begin with circa in the date of birth, for example: "Genghis Khan (c. 1162 – August 18, 1227)". The "c." means that he was born in about 1162, but the exact date is not certain. Sometimes the circa symbol is italicized to show that it is not in the English language.