Circa (often abbreviated c., ca., ca or cca. and sometimes italicized to show it is Latin) means "about" or "around". It is widely used in genealogy and historical writing, when the dates of events are approximately known.
For example, an encyclopedia entry for Genghis Khan may begin with "Genghis Khan (c. 1162 – August 18, 1227)", suggesting that he was born in or around the year 1162. Note that the year of his death appears as a concrete date suggesting that there is definite evidence that this is when he died.
When used in date ranges, a circa is applied before each approximate date, while dates without a circa immediately preceding them are generally assumed to be known with certainty.