Used only when the exact date is not known. If an article says, "Mark Twain's Hannibal treehouse, constructed around 1840," "ca. 1840" is used. But if an article says, "Charlotte Perkins Gilman's mansion built in the 1890s," "1890s" alone is used; no circa is necessary. The same goes for centuries.
Circa (often abbreviated c., ca., ca or cca. and sometimes italicized to show it is Latin) literally means "about". It is widely used in genealogy and historical writing, when the dates of events are approximately known.
The Center for Intructional and Research Computing Activities. CIRCA operates several large VAX computers for classroom use. CIRCA also operates the Faculty Support Center and negotiates site licenses with vendors for instructional and research software.
Same as "about" and "approx."; terms which when used in a letter of credit are construed to allow a difference not to exceed 10% more or 10% less than the monetary amount, or the quantity, or the unit price.