Overscript[x, y] is an object that formats as OverscriptBox[x, y].
Padding
(Built-in Mathematica Symbol) Padding is an option to various array and image operations that specifies what padding to use when extending beyond the original data specified.
PassEventsUp is an option to EventHandler that specifies whether events handled by a particular event handler should be passed up to event handlers in outer expressions.
ResumePacket[] is a MathLink packet used for synchronization with the Mathematica kernel.
Right
(Built-in Mathematica Symbol) Right is a symbol that represents the right-hand side for purposes of alignment and positioning.
StabilityMarginsStyle is an option to frequency response plots such as BodePlot, NyquistPlot, and NicholsPlot that specifies the styles in which the gain and phase margins ...
StringQ
(Built-in Mathematica Symbol) StringQ[expr] gives True if expr is a string, and False otherwise.
x_y^z is the low-level box representation for x_y^z in notebook expressions.
SuspendPacket[] is a MathLink packet used for synchronization with the Mathematica kernel.
f /: lhs =. removes any rules defined for lhs, associated with the symbol f.