NotebookSelection[] represents the current selection in the current evaluation notebook in the front end.NotebookSelection[nb] represents the current selection associated ...
NotElement[x, dom] or x \[NotElement] dom asserts that x is not an element of the domain dom.NotElement[x_1 | ... | x_n, dom] asserts that none of the x_i are elements of ...
NumberMarks is an option for InputForm and related functions that specifies whether ` marks should be included in the printed forms of approximate numbers.
NumberSigns is an option for NumberForm and related functions which gives strings to use as signs for negative and positive numbers.
OneIdentity is an attribute that can be assigned to a symbol f to indicate that f[x], f[f[x]], etc. are all equivalent to x for the purpose of pattern matching.
Operate
(Built-in Mathematica Symbol) Operate[p, f[x, y]] gives p[f][x, y]. Operate[p, expr, n] applies p at level n in the head of expr.
p : v is a pattern object that represents an expression of the form p, which, if omitted, should be replaced by v.
OwnValues[x] gives the rule corresponding to any ownvalue defined for the symbol x.
Pattern
(Built-in Mathematica Symbol) s : obj represents the pattern object obj, assigned the name s.
PlotPoints is an option for plotting functions that specifies how many initial sample points to use.