DefaultButton[] represents an OK button that closes a dialog, and is the default when DynamicBox[ToBoxes[If[$OperatingSystem === "MacOSX", "Return", "Enter"], StandardForm], ...
DynamicSetting[e] represents an object which displays as e, but is interpreted as the dynamically updated current value of Setting[e] if supplied as Mathematica ...
Enabled
(Built-in Mathematica Symbol) Enabled is an option for objects such as Slider that specifies whether the objects should be enabled for interactive manipulation.
Glaisher is Glaisher's constant with numerical value \[TildeEqual] 1.28243.
GraphHighlight is an option to Graph and related objects that specifies graph elements to highlight.
KirchhoffGraph[kmat] gives the graph with Kirchhoff matrix kmat.KirchhoffGraph[{v_1, v_2, ...}, kmat] gives the graph with vertices v_i and Kirchhoff matrix kmat.
Missing
(Built-in Mathematica Symbol) Missing[] represents data that is missing.Missing["reason"] specifies a reason for the data being missing.Missing["reason", expr] associates the expression expr with the ...
NextPrime[n] gives the next prime above n.NextPrime[n, k] gives the k\[Null]^th prime above n.
NotebookRead[notebook] gives the expression corresponding to the current selection in the specified notebook object.
NumberSigns is an option for NumberForm and related functions which gives strings to use as signs for negative and positive numbers.