It is common to use prefixes with numbers for multiples of thousands, such as the ‘kilometre’ for distance and the ‘megatonne’ for nuclear weapons. This is true for sizes in digital imaging as well. Almost from the beginning of modern computers the terms ‘kilobyte’ and ‘megabyte’ have been used for disk sizes, and in more recent times ‘gigabyte’, and now 'terabyte', hard disks have come into usage.
There is a list of these prefixes (see above) with some of them probably being unfamiliar. However, as image file sizes increase, and with them the computer RAM memory plus the storage size of hard disks, then the next prefix on the list will appear. Terabyte hard disks are now available, and at the rate technology is changing then within a few years the first Petabyte disks will arrive.
Although each multiple of a thousand would normally be assumed to be ‘1,000’, this is not exactly the case with digital imaging. Computers work with binary numbers instead of decimal, which is what we use in everyday life. As a consequence the computer has the nearest round figure to a thousand in binary form, which is ‘10000000000’, or ‘1’ followed by 10 noughts. Translated back into decimal figures this equals ‘1,024’. This means that each step of a thousand is actually a multiple of ‘1,024’ instead.