Hyponatremia

Etymology

From Greek "hypo-" (under, below), the Latin-derived "natrium" (sodium, the source of its symbol Na), and Greek "-aemia" (blood condition). The parts spell out the diagnosis exactly: too little sodium in the blood.

noun · /hy-poh-nuh-TREE-mee-uh/
A dangerous condition of abnormally low sodium concentration in the blood.
What it names
The underappreciated flip side of dehydration — the harm of too much water, which can dilute the blood to a lethal degree.
Example
The marathon runner collapsed not from dehydration but from hyponatremia, having drunk so much water that her blood sodium crashed.
Related idea
Hyponatremia is a deficit of sodium relative to water — often from drinking too much; dehydration is a deficit of water itself — the two look similar but call for opposite responses, so mistaking one for the other can be deadly.