The term superset is an word intended to convey the idea of more than the original in a manner opposite the meaning of subset being some portion less than the original.
The term superset within the Tcl community has been used in relationship to programs such as expect, bltsh from BLT, tcl from TclX, and other all-in-one interpreters that have been created at times through Tcl's history. Before the package mechanism became available, one often found that a Tcl extension (aka package) author would include the steps within a Makefile to generate a uniquely named stand alone interpreter which statically linked at least the author's extension (and sometimes a variety of other extensions) into one program. This made it easy for users of Tcl to provide more functionality to a tcl program.
Since the addition of dynamic loading of code and the package mechanism, fewer of these specialty interpreters are maintained (a popular exception to this statement is, of course, Tk's wish).
The stand-alone interpreter might be considered a superset of Tcl because not only did it include Tcl's commands, but it included additional commands.