$InstallationDirectory (Built-in Mathematica Symbol)

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

$IterationLimit (Built-in Mathematica Symbol)

$IterationLimit gives the maximum length of evaluation chain used in trying to evaluate any expression.

$KernelCount (Built-in Mathematica Symbol)

$KernelCount gives the number of subkernels available for parallel computations.

$KernelID (Built-in Mathematica Symbol)

$KernelID is a unique ID number assigned to each running parallel kernel.

$Language (Built-in Mathematica Symbol)

$Language is a list of strings which give the names of languages to use for messages.

$LibraryPath (Built-in Mathematica Symbol)

$LibraryPath gives the default list of directories to search in attempting to find a library.

$LicenseExpirationDate (Built-in Mathematica Symbol)

$LicenseExpirationDate is a date list that gives the expiration date for the license under which Mathematica is being run.

$LicenseID (Built-in Mathematica Symbol)

$LicenseID is a string that gives the license ID under which Mathematica is being run.

$LicenseServer (Built-in Mathematica Symbol)

$LicenseServer is a string that gives the name of the license server that is currently authorizing Mathematica to be run.

$Line (Built-in Mathematica Symbol)

$Line is a global variable that specifies the number of the current input line.