The decade of the 1940s. / Temperatures from 40 to 49 degrees.

plural of 40 (“bottle of beer”)

(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.

(pharmacology) The drug mifepristone.

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

(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, ⁴³₁₉K, having a half-life of 22 hours; used in medicine as a tracer

(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) 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 naturally-occurring radioactive isotope of calcium, ⁴⁸₂₀Ca, having twenty protons and twenty-eight neutrons, and a half-life of 4.3x10¹⁹ years