Wikipedia - Amount of substance (en) Wikipedia - Cantidad de sustancia (es) Wikipedia - Količina supstance (sr) Wikipedia - Quantitat de substància (ca) Wikipedia - Quantità di sostanza (it) Wikipedia - Quantité de matière (fr) Wikipedia - Stoffmenge (de) Wikipedia - Маса (sr) Wikipedia - كمية مادة (ar) Wikipedia - पदार्थ की मात्रा (hi) Wikipedia - 物質量 (ja) amount of substance, $$n$$

Also contains definition of: number of moles
https://doi.org/10.1351/goldbook.A00297
@B00609-1@ in the system of quantities upon which SI is based. It is the number of elementary entities divided by the @A00543@. Since it is proportional to the @N04266@, the proportionality constant being the reciprocal @A00543@ and the same for all substances, it has to be treated almost identically with the @N04266@. Thus the counted elementary entities must always be specified. The words 'of substance' may be replaced by the specification of the entity, for example: amount of chlorine atoms, $$n_{\text{Cl}}$$, amount of chlorine molecules, $$n\left(\text{Cl}_{2}\right)$$. No specification of the entity might lead to ambiguities [amount of sulfur could stand for $$n\left(\text{S}\right)$$, $$n\left(\text{S}_{8}\right)$$, etc.], but in many cases the implied entity is assumed to be known: for molecular compounds it is usually the molecule [e.g. amount of benzene usually means $$n\left(\text{C}_{6}\text{H}_{6}\right)$$], for ionic compounds the simplest formula unit [e.g. amount of sodium chloride usually means $$n\left(\text{NaCl}\right)$$] and for metals the atom [e.g. amount of silver usually stands for $$n\left(\text{Ag}\right)$$]. In some derived quantities the words 'of substance' are also omitted, e.g. @A00295@, @A00296@. Thus in many cases the name of the @B00609-2@ is shortened to amount and to avoid possible confusion with the general meaning of the word the attribute chemical is added. The @C01019@ is hence the alternative name for amount of substance. In the field of clinical chemistry the words 'of substance' should not be omitted and abbreviations such as @S06072@ (for amount of @S06072@) and @S06075@ are in use. The quantity had no name prior to 1969 and was simply referred to as the number of moles.
Sources:
Green Book, 2nd ed., p. 46 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 961 [Terms] [Paper]