虹吸（siphon）是一种流体力学现象，可以不借助泵而抽吸液体。处于较高位置的液体充满一根倒U形的管状结构（称为虹吸管）之后，开口于更低的位置。这种结构下，管子两端的液体压强差能够推动液体越过最高点，向另一端排放，主要是由万有引力让虹吸管作用。2010年5月澳洲昆士兰科技大学休斯博士（Dr Stephen Hughes）说：「是由重力让虹吸管内的液体由上端往下端流动，借由较长且朝下的那一端，将较短上端那一边的水往上引出再流到下端。」

The word siphon (from Ancient Greek: σίφων "pipe, tube", also called syphon) is used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in a narrower sense, the word refers particularly to a tube in an inverted 'U' shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir it came from.