Overlaps is an option to StringCases, StringCount, and StringPosition that specifies how to treat overlapping substrings.
PasteBoxFormInlineCells is an option for cells that specifies whether a new inline cell is created when a typeset expression is pasted into a non-box-type cell.
PrintTemporary[expr] prints expr as a temporary cell in a notebook, removing it when the evaluation of the current input line is complete.
QBinomial[n, m, q] gives the q-binomial coefficient (n; m)_q.
RecordLists is an option for ReadList that specifies whether objects from separate records should be returned in separate sublists.
Riffle
(Built-in Mathematica Symbol) Riffle[{e_1, e_2, ...}, x] gives {e_1, x, e_2, x, ...}. Riffle[{e_1, e_2, ...}, {x_1, x_2, ...}] gives {e_1, x_1, e_2, x_2, ...}. Riffle[list, x, n] yields a list in which ...
RotationAction is an option for three-dimensional graphics functions that specifies how to render 3D objects when they are interactively rotated.
RowSpacings is an option for the low-level function GridBox that specifies the spaces in x heights that should be inserted between successive rows.
SequenceHold is an attribute that specifies that Sequence objects appearing in the arguments of a function should not automatically be flattened out.
ShearingMatrix[\[Theta], v, n] gives the matrix corresponding to shearing by \[Theta] radians along the direction of the vector v, and normal to the vector n.