ArrayDepth[expr] gives the depth to which expr is a full array, with all the parts at a particular level being lists of the same length, or is a SparseArray object.
CellSize is an option for cells that specifies the width and height of an inline cell.
DateTicksFormat is an option for DateListPlot which specifies how date tick labels should be formatted.
DigitBlock is an option for NumberForm and related functions that specifies the maximum length of blocks of digits between breaks.
FileBaseName["file"] gives the base name for a file without its extension.
FileExtension["file"] gives the file extension for a file name.
FileNameSplit["filename"] splits a file name into a list of parts.
FindPermutation[expr] gives a permutation that produces expr by permuting Sort[expr].FindPermutation[expr 1, expr 2] gives a permutation that converts expr_1 to expr_2 for ...
FractionBox[x, y] is a low-level formatting construct that represents x/y in notebook expressions.
GroupActionBase is an option to specify a base for a group.