[Greek hudōr, hudro-, water.]
A hydrant supplies water for fire-fighting; a hydroelectric system generates electricity using flowing water; a hydrofoil is a boat fitted with shaped vanes or foils which lift the hull clear of the water at speed; hydrophobia is fear of water, especially as a symptom of rabies in humans; hydroponics (Greek ponos, labour) is the process of growing plants using liquid nutrients without soil. Hydrocephalus is a condition in which fluid accumulates in the brain. A hydrate is a compound in which water molecules are chemically bound to another compound or an element; hydrolysis is the chemical breakdown of a compound by reaction with water. In some terms, the sense has been broadened to ‘liquid’ or ‘fluid’, as in hydraulic, relating to operation of a machine by a fluid under pressure, or hydrodynamics, the branch of science concerned with forces acting on or exerted by fluids of any type.