RadicalBox[x, n] is a low-level box construct that represents the displayed object Power[x, (n)^-1] in notebook expressions.
RangeFilter[image, r] filters image by replacing every pixel by the difference of the maximum and minimum in its range r neighborhood. RangeFilter[data, r] applies a range ...
RasterSize is an option for Rasterize and related functions that determines the absolute pixel size of the raster generated.
RealBlockDiagonalForm is an option for SchurDecomposition and related functions which specifies whether 2*2 blocks of real values should be used on matrix diagonals in place ...
RecordSeparators is an option for Read, Find, and related functions that specifies the list of strings to be taken as delimiters for records.
ReferenceLineStyle is an option for QuantilePlot and similar functions that specifies the style used for the reference line.
expr //. rules repeatedly performs replacements until expr no longer changes.
Residue
(Built-in Mathematica Symbol) Residue[expr, {z, z_0}] finds the residue of expr at the point z = z_0.
Rest
(Built-in Mathematica Symbol) Rest[expr] gives expr with the first element removed.
Return
(Built-in Mathematica Symbol) Return[expr] returns the value expr from a function. Return[] returns the value Null.