$ModuleNumber (Built-in Mathematica Symbol)

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

$NewMessage (Built-in Mathematica Symbol)

$NewMessage is a global variable which, if set, is applied to the symbol name and tag of messages that are requested but have not yet been defined.

$NewSymbol (Built-in Mathematica Symbol)

$NewSymbol is a global variable which, if set, is applied to the name and context of each new symbol that Mathematica creates.

$Notebooks (Built-in Mathematica Symbol)

$Notebooks is True if Mathematica is being used with a notebook-based front end.

$NumberMarks (Built-in Mathematica Symbol)

$NumberMarks gives the default value for the option NumberMarks, which specifies whether ` marks should be included in the input form representations of approximate numbers.

$Post (Built-in Mathematica Symbol)

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

$RecursionLimit (Built-in Mathematica Symbol)

$RecursionLimit gives the current limit on the number of levels of recursion that Mathematica can use.

$ScheduledTask (Built-in Mathematica Symbol)

$ScheduledTask returns the current ScheduledTaskObject.