MessageDialog[expr] puts up a standard message dialog that displays expr together with an OK button.MessageDialog[expr, {lbl_1 :> act_1, lbl_2 :> act_2, ...}] includes ...

MouseAnnotation[] gives any mouse annotation associated with the expression at the current mouse position.

Names["string"] gives a list of the names of symbols which match the string. Names[patt] gives a list of names matching the arbitrary string pattern patt.

p : v is a pattern object that represents an expression of the form p, which, if omitted, should be replaced by v.

PlotRegion is an option for graphics functions that specifies what region of the final display area a plot should fill.

PrivatePaths -> {opt_1 -> val_1, opt_2 -> val_2, ...} is a global option that specifies settings for paths private to the notebook front end.

Protected is an attribute which prevents any values associated with a symbol from being modified.

RadicalBox[x, n] is a low-level box construct that represents the displayed object Power[x, (n)^-1] in notebook expressions.

RamanujanTauZ[t] gives the Ramanujan tau Z-function Z(t).

RasterSize is an option for Rasterize and related functions that determines the absolute pixel size of the raster generated.