(physics) An artificially produced radioactive isotope of potassium, ⁴³₁₉K, having a half-life of 22 hours; used in medicine as a tracer

(US Army and Marines, military, slang) Any weapon firing a 40mm grenade. Often specifically the M203 grenade launcher, mounted underneath an M-16 or variant. In the Marine Corps, often the Mark-19 Automatic 40mm Grenade Launcher.

(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

(physics) An artificially produced radioactive isotope of potassium, ⁴²₁₉K, having a half-life of 12.4 hours; used in medicine as a tracer

(physics) the only stable isotope of scandium, ⁴⁵₂₁Sc, having twenty-one protons and twenty-four neutrons

(physics) A minor stable isotope of titanium, ⁴⁷₂₂Ti, having twenty-two protons and twenty-five neutrons; it amounts to about 7% of the element in nature.

(physics) A minor stable isotope of titanium, ⁴⁶₂₂Ti, having twenty-two protons and twenty-four neutrons; it amounts to about 8% of the element in nature

(physics) A minor stable isotope of calcium, ⁴⁴₂₀Ca, having twenty protons and twenty-four neutrons; it amounts to about 2% of the element in nature

(physics) A minor stable isotope of calcium, ⁴²₂₀Ca, having twenty protons and twenty-two neutrons; it amounts to about 0.6% of the element in nature

A penetrating oil used as a lubricant and to protect electric circuits from moisture.