Bob:
Originally called Duck Tape when first developed, (See
http://www.ducktapeclub.com/) it has become known incorrectly as duct tape. (See also
http://www.octanecreative.com/ducttape/duckvsduct.html)
Morty
PS: What folks now refer to as "duct" tape should not be used on ducts at all (as any HVAC guy will tell you) because inevitably it dries out and falls off. What they use is silver tape.