pressure, p in gas chromatography

https://dx.doi.org/10.1351/goldbook.P04819
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 pressurep i = x i p, where x i is the amount fraction of the ith constituent and p is the total pressure.
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 987