Not to be confused with Psiphon.
This article is about the domestic device. For the animal organ, see siphon (disambiguation).
For the carbonation device, see soda siphon.

The word siphon (from Ancient Greek: σίφων "pipe, tube", also spelled syphon) is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes - see siphon terminology - but in the narrower sense it refers specifically to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir, without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity, and is discharged at a level lower than the surface of the reservoir it came from. Note that while the siphon must touch the liquid in the (upper) reservoir (the surface of the liquid must be above the intake opening), it need not touch the liquid in the lower reservoir and indeed there need not be a lower reservoir - liquid can discharge into mid-air.
In practical siphons, atmospheric pressure pushes the liquid up the tube into the region of reduced pressure at the top of the tube in the same way as a manometer, and indeed the maximum height of a siphon is the same as the height of a manometer exposed to the atmosphere, because they operate by the same mechanism. The reduced pressure is caused by liquid falling on the exit side.
When both ends of a siphon are at atmospheric pressure, liquid flows from high to low. However, if the lower end is pressurized, liquid can flow from low to high, as in siphon coffee. While in everyday siphons, atmospheric pressure is the driving mechanism, in specialized circumstances other mechanisms can work - in the laboratory, some siphons have been demonstrated to work in a vacuum - see vacuum siphons - indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures. Most familiar siphons have water as a fluid, though mercury is often used in experiments, and other fluids such as organic liquids or even carbon dioxide can be siphoned.