Source : Free On-Line Dictionary of Computing

eight-bit clean
     
         A term which describes a system that deals
        correctly with extended {character sets} which (unlike ASCII)
        use all eight {bits} of a {byte}.  Many programs and
        communications systems assume that all characters have codes
        in the range 0 to 127.  This leaves the top bit of each byte
        free for use as a {parity} bit or some kind of {flag bit}.
        These assumptions break down when the program is used in some
        non-english-speaking countries with larger alphabets.
     
        If a binary file is transmitted via a communications link
        which is not eight-bit clean, it will be corrupted.  To combat
        this you can encode it with {uuencode} which uses only {ASCII}
        characters.  There are some links however which are not even
        "seven-bit clean" and cause problems even for uuencoded data.
     
        (1995-01-05)