$InitialDirectory gives the initial directory when the current Mathematica session was started.
$InitialDirectory returns the full name of the directory as a string.
If Mathematica is started from a shell or command line, $InitialDirectory gives the current operating system directory.
If Mathematica is started from a menu or icon, $InitialDirectory typically gives the user's home directory $HomeDirectory.
See Section 2.12.4.
See also: Directory, $CommandLine, $HomeDirectory, $BaseDirectory, $InstallationDirectory.
New in Version 3; modified in 5.0.