"Circa" means "at" or "about," so in connection to a salary, it means "about" or "near" a certain salary. "Circa" is often used when referring to the dates of historical events that are only approximate, such as "the battle happened circa 1400."
The English word "circa" comes from the Latin "circa," which means "around"; "circa" is a variation of the Latin "circum." "Circum" has been carried into English in words like "circumvent" (going around an object), "circumstantial" (a non-essential part of something, link circumstantial evidence) and "circumference" (the distance around the outside of an object). "Circa" did not enter the English vernacular until the mid-19th century.
