The word "circa" is a preposition that means "at approximately," "in approximately," or "of approximately," and it is used especially with dates when the exact time is unknown. For example, saying someone was born "circa 1600" means they were born around the year 1600, but the exact date is not known. It originates from Latin, meaning "around" or "near," which ties to the idea of approximation or estimation of time. It is often abbreviated as "c." or "ca." and is commonly used in historical or archaeological contexts to give an estimated date rather than a precise one.

In summary, "circa" indicates an approximate or around date or time, typically used when precise information is unavailable but an estimated time frame is known.