hydrology 1. the height of water above any plane of reference 2. the kinetic or potential energy possessed by each unit weight of a liquid, expressed as the vertical height through which a unit weight would have to fall to release the average energy possessed —note head is used in various compound terms, such as pressure head, velocity head, and lost head 3. the internal pressure expressed in ft or lb/in² (m or g/cm²) of an enclosed conduit 4. fire —see forest fireThis definition last updated 10/27/2008