A gramophone record played at 45 revolutions per minute. / (US politics, slang, uncountable) Donald Trump, as the forty-fifth president of the United States.

(US, slang) A bottle containing 40 fluid ounces of malt liquor beer.

(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

(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) 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 radioactive isotope of calcium, ⁴⁵₂₀Ca, having twenty protons and twenty-five neutrons, and a half-life of 162.6 days; it is used as a tracer in studying calcium metabolism. As it decays it undergoes transformation (nuclear transmutation) via electron emission, also called negative beta decay (β−-decay), into scandium.

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