The term that hardware manufacturers mean when they say "gigabyte". A marketing gigabyte is only 1 billion bytes. A real gigabyte is 1,000,048,576 kilobytes/1.073741e+09 bytes. If you read the fine print you will see a little notice about how they redefined gigabyte.
They do this to make their drives seem larger. So when you go and buy a 20 gigabyte hard drive, you are probably only getting a ~19.5 gigabyte hard drive.