Becuase it has two pieces, is one bikini referred to as "a bikini" or "a pair of bikinis"?

Question

and does it make a difference in US Grammar vs UK Grammar and Australian Grammar?

Answer 1

You're misunderstanding the word. "Bikini" does not begin with the prefix "bi-" (meaning "two"). It is derived from the name of an atoll in the Marshall Islands (Pacific Ocean), famous after an atom-bomb test was conducted there in June 1946 (though it not quite clear how exactly this gave rise to the name for the swim-wear, which appeared first in French in 1947).

So you simply call the WHOLE thing (the two pieces taken together) "a bikini", never a "pair" of anything. If you need to refer to the separate parts they are "bikini top" and "bikini bottom".

http://www.etymonline.com/index.php?search=bikini
http://www.thefreedictionary.com/bikini

Answer 2

I think you want to refer as
scissors
shoes
pants....?
Well.
bikini has two pieces for 2 breasts?
then ok to say bikinis..haha