ActiveTcl uses the term base-tcl in a fashion similar to tclkit.
A base-tcl interpreter is a stand alone, Tcl only, interpreter which includes the following package names:
So there are a number of differences in terms of what packages one will find by default in a base-tcl.
Anyone have information on other differences? One thing to notice is that Tk isn't in the list base. For that package, check on base-tk, which is another application in ActiveTcl.