SquareSupersetEqual[x, y, ...] displays as x \[SquareSupersetEqual] y \[SquareSupersetEqual] ....
StringSkeleton[n] represents a sequence of n omitted characters in a string printed with Short. The standard print form for StringSkeleton is an ellipsis.
StyleDefinitions is an option for notebooks that gives definitions for the styles that can be used in a notebook.
SuperscriptBox[x, y] is the low-level box representation for x^y in notebook expressions.
UnderscriptBox[x, y] is the low-level box representation for UnderscriptBox[x, y] in notebook expressions.
VerticalTilde[x, y, ...] displays as x\[VerticalTilde]y\[VerticalTilde]....
WindowSize is a notebook option that specifies the size of window that should be used to display a notebook on the screen.
$Canceled is a symbol returned when notebook input is canceled, for example from a dialog box.
$ConfiguredKernels is the default list of kernels that are configured to be launched for parallel computing.
$CurrentLink is the LinkObject representing the MathLink connection for an external program currently being installed or being called.