Noun. 1. siphon-a tube running from the liquid in a vessel to a lower level outside the vessel so that atmospheric pressure forces the liquid through the tube. syphon.tube, tubing-conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases.. A pipe or tube in the form of an upside-down U, filled with liquid and arranged so that the pressure of the atmosphere forces liquid to flow upward from a container through the tube, over a barrier, and into a lower container. 2. A tubular animal part, as of a clam, through which water is taken in or expelled.