$MaxPrecision (Built-in Mathematica Symbol)

$MaxPrecision gives the maximum number of digits of precision to be allowed in arbitrary-precision numbers.

$MessageList (Built-in Mathematica Symbol)

$MessageList is a global variable that gives a list of the names of messages generated during the evaluation of the current input line.

$MinNumber (Built-in Mathematica Symbol)

$MinNumber gives the minimum positive arbitrary-precision number that can be represented on a particular computer system.

$ModuleNumber (Built-in Mathematica Symbol)

$ModuleNumber gives the current serial number to be used for local variables that are created.

$Path (Built-in Mathematica Symbol)

$Path gives the default list of directories to search in attempting to find an external file.

$Post (Built-in Mathematica Symbol)

$Post is a global variable whose value, if set, is applied to every output expression.

$Pre (Built-in Mathematica Symbol)

$Pre is a global variable whose value, if set, is applied to every input expression.

$PrePrint (Built-in Mathematica Symbol)

$PrePrint is a global variable whose value, if set, is applied to every expression before it is printed.

$PreRead (Built-in Mathematica Symbol)

$PreRead is a global variable whose value, if set, is applied to the text or box form of every input expression before it is fed to Mathematica.

$TemporaryDirectory (Built-in Mathematica Symbol)

$TemporaryDirectory gives the main system directory for temporary files on your computer system.