The amount of substance, symbol \(n\), of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
This wording is part of the definition of the SI base unit mole, which was agreed upon by the 26th CGPM in November 2018 with effect from 20 May 2019.
BIPM, Le Système international d’unités (SI), SI Brochure, 9e ed. (2019), p. 21 [Terms] [Book]
BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 134 [Terms] [Book]
PAC, 2018, 90, 175. (Definition of the mole) [Terms] [Paper]