An Etymological Dictionary of Astronomy and Astrophysics
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The number of atoms in 12 grams of ¹²C; by extension, the number of atoms in a gram-atom (or the number of molecules in a → mole) of any substance: 6.02 × 10²³.

Named after Amedeo Avogadro (1776-1856), whose law allowed other physicists to calculate Avogadro's number; → number.