(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 rare stable isotope of calcium, ⁴⁶₂₀Ca, having twenty protons and twenty-six neutrons; it amounts to only 0.004% of the element in nature

(music, radio) A radio format for popular music, usually including the 40 most popular songs at a given time.

(physics) A minor stable isotope of calcium, ⁴³₂₀Ca, having twenty protons and twenty-three neutrons; it amounts to about 0.1% 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

(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 titanium, ⁴⁸₂₂Ti, having twenty-two protons and twenty-six neutrons; it amounts to about 74% of the element in nature

US standard spelling of 400 metres.

Someone who carries out a Nigerian scam.