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 scripts or the Tcl [C] [API]. An extension may define new actions, new [widget]s, new processing types, etc. ---- You might also want to see an [extension example]. ---- [Category Glossary]