means The process of taking in and retaining enough water to keep the body's cells and systems working properly.

from From the Greek 'hydor,' meaning water — the same root that pours through 'hydrant,' 'hydrogen' (literally 'water-former'), and the many-headed 'hydra' lurking in its swamp. Chemists in the early 19th century borrowed 'hydrate' for compounds that bind water, and the noun 'hydration' followed as the act of doing so. The everyday sense — drink your water — is a much more recent migration of the lab word into the gym and the water bottle.