Dehydration

Etymology

From "de-" (removal, reversal) and Greek "hydor" (water), plus "-ation." The "hydro-" root runs through "hydrant," "hydrogen," and "hydraulic" — all circling water. Dehydration is literally the taking-away of water.

noun · /dee-hy-DRAY-shun/
A shortfall of water in the body serious enough to impair how it works.
What it names
A handle for the state of running low on fluid — not just feeling thirsty but the body actually short of what it needs.
Example
On the long hike the first sign of dehydration wasn’t thirst but a dull headache.
Related idea
Thirst lags behind actual dehydration — by the time you feel thirsty you’re often already mildly depleted, which is why endurance advice says to drink on a schedule, not on the urge. Its opposite, over-drinking to the point of diluting the blood’s salt, is hyponatremia.