prior distribution, \(P_{0}\)

https://doi.org/10.1351/goldbook.P04852
A product state distribution calculated on the basis of some physical model for the reaction. The distribution usually referred to is that of the products at the instant of their formation.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 178 (https://doi.org/10.1351/pac199668010149)