Siphon/ˈsaɪfn/ Definition 1 of 3Pro Tip 1/3Gravity Dependent The noun form of 'siphon' relies on gravity to work, like sucking liquid through a straw. The mechanic needed a siphon to empty the gasoline tank. Pro Tip 2/3Flow Direction 'Siphon' implies a liquid moving from a higher area to a lower one without a pump. After the rain, we used a siphon to remove water from the cover. Pro Tip 3/3Initiating Process Starting a siphon may require suction to begin the liquid flow. He started the siphon by sucking on the tube.