Mathematica allows arbitrary hierachical namespaces. However, several standard "contexts" are conventionally used for predefined purposes.
Unicode: 00AE. Alias: Esc rtm Esc. Letter-like form. Used as a superscript to indicate a registered trademark such as Mathematica. Typically used only on the first occurrence ...
Unicode: 21A6. Infix arrow operator. x ↦ y is by default interpreted as RightTeeArrow[x,y]. Used in mathematics to indicate a transformation, often the action of a mapping on ...
File system directory hierarchy. Can contain files and subdirectories.
"CellLabelsToTags" is a front end token that gives a selected cell a cell tag that is the same as its current input or output prompt.
"CompleteSelection" is a front end token that opens a popup menu to complete a partially typed word.
"EvaluatorAbort" is a front end token that aborts the kernel operation in the specified kernel.
"EvaluatorQuit" is a front end token that terminates the specified kernel.
"EvaluatorStart" is a front end token that starts the specified kernel.
"FileNameDialog" is a front end token that opens a dialog for inserting a file path.