The Free On-line Dictionary of Computing (30 December 2018):

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)