Tk is short for toolkit. It's the cross-platform GUI toolkit (for X11 and Windows among others) used by Tcl (people often refer to this combination as Tcl/Tk). It was created by Tcl's creator Dr. John Ousterhout.

Its first release happened in 1991, while development of Tcl started in 1989.

While Tk has Tcl origins, it also has bindings in other languages (Python and Perl being the most important ones - these bindings are pretty important because Tk is a decent-looking toolkit but Tcl, I've heard, stinks. =)

Sources used: "The Linux Tcl and Tk HOWTO"