Source : Free On-Line Dictionary of Computing

standard input/output
     
         The predefined input/output
        channels which every {Unix} process is initialised with.
        Standard input is by default from the terminal, and standard
        output and standard error are to the terminal.  Each of these
        channels (controlled via a {file descriptor} 0, 1, or 2 -
        stdin, stdout, stderr) can be redirected to a file, another
        device or a {pipe} connecting its process to another process.
        The process is normally unaware of such {I/O redirection},
        thus simplifying prototyping of combinations of commands.
     
        The {C} programming language library includes routines to
        perform basic operations on standard I/O.  Examples are
        "printf", allowing text to be sent to standard output, and
        "scanf", allowing the program to read from standard input.
     
        (1996-06-07)