What is Isochron?

Isochron meaning in Planetary Science terminology / glossary / dictionary is:
In radiometric age dating by isotopic analyses: The line in an isochron diagram, whose slope is proportional to the amount of time passed (it defines the crystallization age) and whose intercept gives the initial isotopic ratio of the sample when it crystallized. In relative age dating by crater counting: The line on a log-log plot (of cumulative number of craters per km2 versus crater diameter in kilometers) indicating the size distribution of impact craters found on a surface of a specified age, if no other processes have obliterated or altered the surface.


reference: Planetary Science Research. PSRD Discoveries