isochron

A line on a chart linking rock of the same age (as determined from isotope ratios).

Recapitulating, any suite of comagmatic rocks with an age t defines an isochron if its members all had the same initial ^{₈₇}Sr/^{₈₆}Sr ratio and if it was closed to both Rb and Sr since crystallization occurred and neither element was introduced from any extraneous source.[…]In this way, whole-rock Rb-Sr isochrons can produce rather reliable age data for rock samples.

A collection of points (such as a manifold), each point representing a set of initial conditions for a given dynamical system, such that every set results in the same long-term behaviour of the system.

The entire plane is foliated by isochrons.