Source : Free On-Line Dictionary of Computing

file descriptor
     
         An integer that identifies an
        open {file} within a {process}.  This number is obtained as a
        result of opening a file.  Operations which read, write, or
        close a file would take the file descriptor as an input
        parameter.
     
        In many {operating system} implementations, file descriptors
        are small integers which index a table of open files.  In
        {Unix}, file descriptors 0, 1 and 2 correspond to the
        {standard input}, {standard output} and {standard error} files
        respectively.
     
        See {file descriptor leak}.
     
        (1998-02-06)