point of departure

noun3

1. •
   (formal) A starting point, either a physical place or the initial idea or fact from which a discussion, journey, project, or analysis begins.

   • […] so “going back to the way things were” is not a useful point of departure. — 2021
2. •
   Synonym of starting point.

   • […] the point of departure is Hong Kong International Airport. — 2021
3. •
   A dosage level at which additional administration of an agent significantly loses its efficacy.

   • In toxicology, point of departure (POD) is defined as the point on a toxicological dose-response curve […] generally corresponding to an estimated low effect level or no effect level. — 2017