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.
It is fairly common to use the term "extension" to mean only things that are not pure Tcl.
An extension may define new actions, new widgets, new processing types, etc.
You might also want to see an extension example, as well as the notion of "extension language".