These days, a generic term that applies to many variations of the original Bell Labs operating system - includes Linux in its infinite variations as well as Solaris, HP-UX, etc. The Mother Wiki has a good page [L1 ] on the subject.
LES: Perhaps it should be noted that Unix is not the same as Linux. It is very easy to find support to all things Tcl in Linux, but not so easy in the BSD Unix family. The point is, some things will work fine in one, but not in the other. Especially binary things, like Tclkit, Freewrap and certainly a few extensions. These would have to compiled for Unix specifically, maybe after a few modifications.