ˈsaɪfən ˈsaɪfən a tubular organ in an aquatic animal (especially in mollusks) through which water can be taken in or expelled move a liquid from one container into another by means of a siphon or a siphoning action siphon gas into the tank convey, draw off, or empty by or as if by a siphon a tube running from the liquid in a vessel to a lower level outside the vessel so that gravity forces the liquid through the tube