Number of particles divided by the volume they occupy.
Sources:
Green Book, 2nd ed., p. 39 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2183 (https://doi.org/10.1351/pac199062112167)