Well, anyway, I let this thing get a bit off track. Whatever the origin of the term "double bass", there seems to be a consensus that it's always been the same thing, whether called "contrabass" or "double bass" - in other words, the lowest member of the violin family, lower than what we now call the "cello". There has been a huge amount of variation in the physical characteristics of these things, but as far as I know, the two names have always been applied to the same set of instruments.