stdin

from The Free On-line Dictionary of Computing (8 July 2008)
standard input/output
standard I/O
stderr
stdin
stdio
stdout

   <programming, operating system> 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)