[Yo-Yo]

I disagree and believe the original definition is correct.

Gravity causes liquid in the lower leg to fall, lowering the pressure above it, causing atmospheric pressure to force liquid from the reservoir up into the shorter leg.

The longer leg of liquid causes a continuous “sucking” on the column of liquid above the shorter leg.

[BitWielder1]

Right. A siphon will not work in vacuum, it takes external atmospheric pressure to push the liquid up the shorter leg.
Gravity and atmosphere will keep the process going, once it started.

[sima_yi]

The original dictionary definition is incorrect (incomplete), and Prof. Hughes definition is also incorrect (incomplete).

Your explanation is, in fact, the correct description of the process. Gravity causes the liquid in the long end of the tube to fall, which creates a pressure differential along the length of the tube, which atmospheric pressure on the to-be-emptied liquid attempts to "correct".

[Oberon]

You’re right. The power of the siphon is limited to the amount of atmospheric pressure in which it’s operated, i.e. about 14.7 psi at sea level.

[HerrBlucher]

Would a siphon work in zero G conditions? Has this ever been tested in space? If the atmospheric pressure theory is correct the siphon should work in the zero g artificial atmosphere of a spaceship. If not, then one can only conclude that gravity is the culprit.

[Oztrich Boy]

OED: 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.

Is correct, Siphon would not work in a vacumn, or if the shorter leg was more than 34' at normal Atmospheric pressure.