Normal force acting on a surface divided by the area of that surface. For a mixture of gases the contribution by each constituent is called the partial pressure p i = x i p, where x i is the amount fraction
x (
y for gaseous mixtures) of the ith constituent and p is the total pressure.
Sources:
Green Book, 2nd ed., p. 12 (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 987 (https://doi.org/10.1351/pac199668040957)