A distributor that works by adjusting its speed based on the amount of vacuum coming from the intake manifold. Stock Type I Volkswagens had them installed. They are generally accepted to be inferior to the centrifugal advance distributor.

On a Type I Volkswagen you can check if it is working by taking off the distributor cap and pulling the cable off the intake manifold and sucking on it. If the gap between the contacts moves, then it's working. (I shouldn't have to tell you to turn the engine off before you do this).