An extension is a common term meant to convey an increased functionality (i.e. extending functionality) and is used to refer to code that defines additional [Tcl] procs either by way of [package] and Tcl [script]s or the Tcl [C] [API].

An extension may define new actions, new [widget]s, new processing types, etc.


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You might also want to see an [extension example],
as well as the notion of "[extension language]".
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[Category Glossary]