Potassium-argon Dating Definition

pōtasēəmärgän, pətasēəmärgän
noun
An indirect method of dating fossils, esp. those in very ancient volcanic rock, by using potassic minerals in the same strata as a reference: a natural radioactive isotope of potassium, K-40, decays steadily into argon, which accumulates in the minerals and is measured to determine age.
Webster's New World