The OED's definition:

A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.

Naturally, we double-checked our entry. Here's the definition of siphon from the Collegiate, 11th Edition:

a tube bent to form two legs of unequal length by which a liquid can be transferred to a lower level over an intermediate elevation by the pressure of the atmosphere in forcing the liquid up the shorter branch of the tube immersed in it while the excess of weight of the liquid in the longer branch when once filled causes a continuous flow.

Note that the second part of this definition clearly indicates the role that gravity plays in the process by causing the continuous flow of liquid. This does not conflict with the claims of Dr. Hughes, but it does recognize the role that atmospheric pressure plays in initializing the process.

Merriam-Webster editors are satisfied our definition is correct and adequate, but we will follow the debate sparked by the siphon controversy. If the results indicate that a change to emphasize the role that gravity plays is necessary, we will revise our entry accordingly.