$InstallationDirectory (Built-in Mathematica Symbol)

$InstallationDirectory gives the top-level directory in which your Mathematica installation resides.

init.m (Mathematica File)

init.m is a Mathematica initialization file.

$InitialDirectory (Built-in Mathematica Symbol)

$InitialDirectory gives the initial directory when the current Mathematica session was started.

$UserName (Built-in Mathematica Symbol)

$UserName gives the login name of the user who invoked the Mathematica kernel, as recorded by the operating system.

CharacterEncodingsPath (Built-in Mathematica Symbol)

CharacterEncodingsPath is a global option that specifies which directories are searched for character encoding files.

PalettePath (Built-in Mathematica Symbol)

PalettePath is a global option that specifies which directories Mathematica searches for palettes on startup.

SpellingDictionariesPath (Built-in Mathematica Symbol)

SpellingDictionariesPath is a global option that specifies which directories are searched for spelling dictionaries when the Edit \[FilledRightTriangle] Check Spelling menu ...

StyleSheetPath (Built-in Mathematica Symbol)

StyleSheetPath is a global option that specifies which directories Mathematica searches to find stylesheets.

Systemwide Defaults (Mathematica Tutorial)

If you have installed Mathematica in a location where multiple users can run it (for example, on a file server or multi-user machine), then you can set up systemwide defaults ...

$BaseDirectory (Built-in Mathematica Symbol)

$BaseDirectory gives the base directory in which systemwide files to be loaded by Mathematica are conventionally placed.