$ParentLink (Built-in Mathematica Symbol)

$ParentLink is the MathLink LinkObject currently used for input and output by the Mathematica kernel in a particular session.

$ParentProcessID (Built-in Mathematica Symbol)

$ParentProcessID gives the ID assigned to the process which invokes the Mathematica kernel by the operating system under which it is run.

$PathnameSeparator (Built-in Mathematica Symbol)

$PathnameSeparator is a string used as a separator when full file and directory names are constructed.

$Path (Built-in Mathematica Symbol)

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

$PerformanceGoal (Built-in Mathematica Symbol)

$PerformanceGoal gives the default setting for the option PerformanceGoal for graphics and other algorithmic functions.

$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.

$ProcessID (Built-in Mathematica Symbol)

$ProcessID gives the ID assigned to the Mathematica kernel process by the operating system under which it is run.