RhymeZone: potassium-40 definitions

Definitions of potassium-40:

noun: (physics) A naturally-occurring radioactive isotope of potassium, ⁴⁰₁₉K, having nineteen protons and twenty-one neutrons, and a half-life of 1.3 billion years; it is the chief source of natural radioactivity in living tissue

(Definitions from Wiktionary)