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hydra (usu. cap.) in Greek mythology, the serpent with nine heads, of which one was immortal, slain by Hercules. [4 definitions]
hydrangea any of various shrubby plants that are grown for their large round or flat clusters of blue, pink, or white blossoms. [2 definitions]
hydrant an upright pipe with a valve from which water can be drawn from a water main.
hydrate a compound that contains water in chemical combination. [2 definitions]
hydraulic of, concerning, operated by, or moved by water or another liquid under pressure. [3 definitions]
hydraulic fracturing the injection into an oil or natural gas well of large amounts of pressurized water containing sand and chemicals, which fractures the surrounding rock, creating fissures through which oil or gas flows into the well; fracking.
hydraulic ram a water pump or device using the energy of downward flowing water to lift a part of the water for some other use.
hydraulics (used with a sing. verb) the science or technology concerned with the mechanical uses of fluids, esp. water, in motion.
hydrazine a colorless and corrosive fuming liquid substance of nitrogen and hydrogen that is used in rocket and jet fuels.
hydric of, relating to, or preferring a moist or wet environment. [2 definitions]
hydride an inorganic compound, such as sodium hydride, containing hydrogen and a second usu. electropositive element or group.
hydro- water. [2 definitions]
hydrocarbon any of various organic compounds that contain only hydrogen and carbon, such as methane or ethylene.
hydrocephalus a condition, esp. in children, in which an abnormal amount of serum accumulates in the brain's ventricles, causing the skull to enlarge and the brain to compress.
hydrochloric acid a strong poisonous acid of hydrogen and chlorine in a water solution, esp. used for industrial and chemical purposes.
hydrochloride a salt formed by the combination of hydrochloric acid and an organic base.
hydrocortisone the hormone cortisol used pharmaceutically.
hydrocyanic acid a highly poisonous, flammable solution of hydrogen cyanide in water, used in manufacturing, working with metals, and the like; prussic acid.
hydrodynamics (used with a sing. verb) the science concerned with the motion of fluids, esp. as it relates to solid objects immersed in fluid, or with the motion of solid objects through fluids.
hydroelectric of or concerning the generation of electricity by means of the energy produced by moving water.
hydrofluoric acid a highly poisonous water solution of hydrogen fluoride, colorless and corrosive, used esp. in etching glass.