$BatchOutput is True if output in the current session is being sent in batch mode, suitable for reading by other programs.
The initial value of $BatchOutput is typically determined by a command-line option when the Mathematica session is started.
$BatchOutput cannot be reset during a Mathematica session.
When $BatchOutput is set to True, Mathematica generates all output in InputForm, with the PageWidth option effectively set to Infinity, does not give In and Out labels, and does not give any banner when it starts up.
See The Mathematica Book: Section 2.13.5.
See also: $BatchInput, $Linked, $CommandLine.