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Middle English, from Old English wæter; akin to Old High German wazzar water, Greek hydōr, Latin unda wave


  • 1 a : the liquid that descends from the clouds as rain, forms streams, lakes, and seas, and is a major constituent of all living matter and that when pure is an odorless, tasteless, very slightly compressible liquid oxide of hydrogen H2O which appears bluish in thick layers, freezes at 0° C and boils at 100° C, has a maximum density at 4° C and a high specific heat, is feebly ionized to hydrogen and hydroxyl ions, and is a poor conductor of electricity and a good solvent
b : a natural mineral water —usually used in plural
  • 2 : a particular quantity or body of water: as a (1) plural : the water occupying or flowing in a particular bed (2) chiefly British : lake, pond
b : a quantity or depth of water adequate for some purpose (as navigation)
c plural (1) : a band of seawater abutting on the land of a particular sovereignty and under the control of that sovereignty (2) : the sea of a particular part of the earth
d : water supply <threatened to turn off the water>
  • 3 : travel or transportation on water <we went by water>
  • 4 : the level of water at a particular state of the tide : tide
  • 5 : liquid containing or resembling water: as a (1) : a pharmaceutical or cosmetic preparation made with water (2) : a watery solution of a gaseous or readily volatile substance — compare ammonia water
b archaic : a distilled fluid (as an essence); especially : a distilled alcoholic liquor
c : a watery fluid (as tears, urine, or sap) formed or circulating in a living body
d : amniotic fluid; also : bag of waters
b : degree of excellence <a scholar of the first water>
  • 7 : watercolor
  • 8 a : stock not representing assets of the issuing company and not backed by earning power
b : fictitious or exaggerated asset entries that give a stock an unrealistic book value


Water is a chemical substance that is composed of hydrogen and oxygen and is vital for all known forms of life.

In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71% of the Earth's surface. On Earth, it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. A very small amount of the Earth's water is contained within biological bodies and manufactured products.

Water on Earth moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually reaching the sea. Over land, evaporation and transpiration contribute to the precipitation over land.

Clean drinking water is essential to human and other lifeforms. Access to safe drinking water has improved steadily and substantially over the last decades in almost every part of the world. There is a clear correlation between access to safe water and GDP per capita. However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability. A recent report (November 2009) suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%. Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of freshwater is consumed by agriculture.[1]