Originally, it was called duck tape because the cloth is/was duck. I don't know when the term changed, but it might have something to do with not using duck anymore. (Most of the duct tape I've seen has used a really loose cloth. Gaffer's tape, on the other hand, is still tightly woven.) Of course, this all brings up the question of if you'd use duct tape to tape ducts. It's about the worst tape to use for that.