The original operating system for the Apple [L1 ] Macintosh Computer (which was not the first computer from Apple - it was actually a fourth or more generation computer). Also called MacOS Classic to differentiate from MacOS X.
These days, most Macs are coming with MacOS X installed.
Note however that Daniel Steffen and others continue to provide Tcl, Tk, and even Tclkit in formats suitable for MacOS Classic (both 68k and Power PC!).
DKF vaguely remembers the Apple ][, which is kind-of awkward to type in the Wiki... LV has several models of these still - though that is likely to change in the upcoming weeks.
See QuickTimeTcl as one of the Mac specific extensions.