Group of four quantities characterizing the composition of a mixture with respect to the volume of the mixture (mass, amount, volume and number concentration).
Short form for amount (of substance) concentration (substance concentration in clinical chemistry).
Sources: Green Book, 2nd ed., p. 42 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf) PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 966 (https://doi.org/10.1351/pac199668040957)
See also: PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2181 [Terms] [Paper]