I don't think there's a real definition to duct or gaffer tape, both got named for very unspecific reasons. Duct tape because it was used on ducting and gaffer tape because the Gaffer (head electrician in the film industry) used it.
McBryce.
No, actually, the thing commonly called "duct tape" (or my favorite, NASCAR tape) was originally created for the US Military by Anderson tape MFG during WWII, on request for a waterproof sealing tape that could be used for temporary repair of tents and tarps, etc. This came to be known as "Duck tape" from the saying that it sealed "tighter than a duck's arse, and that's watertight", and would even seal when wet. It came to be used for sealing metal ductwork after the war, and then came about the whole "Duck Tape/Duct Tape" argument.
During the evolution of the companies involved, different colors evolved from the original GI Tan & Olive Drab to grey and silver for its popular use in ductwork, to black/white/blue for its use in music, theatrical and film productions. It was sometime around the age of Star Wars that descendant company MANCO made the "Duck Tape" name official and introduced a variety of colors for the consumer market.
mnem
Tape that duck!!!