It should be blindingly obvious - both from the word itself and from the context - that "circa" (often shortened to c.) means "around" or "about", as in Roman ruins, circa 150 AD or Genghis Khan, born Mongolia, c. 1162. Look it up in a dictionary (or, if you must, Wikipedia).
Ignorant Australian estate agents have gotten hold of this word over the last few years and corrupted it, like they do to so much of the language. Now we get ridiculous things in ads for houses like "Art Deco masterpiece, circa 1980s".
Memo to land-rats (1): The word "circa" and a date like "1980s" are completely incompatible. You could look up the records and find exactly - to the day - when the certificate of occupancy was issued, or whatever other marker of construction you wished to choose. There's no bloody "circa" about it... unlike the date of birth of Genghis Khan, for which no certificate or other documentary evidence exists and we have to make do with a best guess, i.e. a year or two either side of 1162.
And don't get me started on "Art Deco". I have seen everything from Victorian to the 1980s (I kid you not) described by real estate agents as "Art Deco". And search eBay for "art deco" and you'll see 1950s and even 1960s furniture described this way. (NB The picture shows an "art deco" boardroom table! Not in my deco books...)
Memo to land-rats (2): Don't use words unless you know what they really mean.
Of course, the real problem is that so many people believe what they see and hear, even if it is from an estate agent, and these two dumbed-down uses are undoubtedly gaining traction. Once they enter the mainstream, we'll be stuck with 'em.