(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) The major stable isotope of argon, ⁴⁰₁₈Ar, having eighteen protons and twenty-two neutrons; it amounts to about 99.6% of the element in nature

(physics) The major stable isotope of calcium, ⁴⁰₂₀Ca, having twenty protons and twenty neutrons; it amounts to almost 97% of the element in nature

(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.

The geographic region between 10 degrees north and 40 degrees north latitude, especially when considered as a target for Christian missionary work.

(athletics) A sprint race over 400 metres.

simple past tense and past participle of 404

plural of 404 page

(Internet) The result of trying to follow a broken link within a website.

(US) A tax-qualified, defined-contribution pension account.