potassium

noun

Etymology: Coined by British chemist Humphry Davy in 1807, from potassa (a Latinized form of potash) + -ium.

1. A soft, waxy, silvery reactive metal that is never found unbound in nature; an element with atomic number 19 and atomic weight of 39.0983.

   “We all need potassium, and green vegetables are a good source.”

2. An atom of this element.

   “This reaction substitutes a potassium for a sodium.”