Isochron

In the mathematical theory of dynamical systems, an isochron is a set of initial conditions for the system that all lead to the same long-term behaviour.

Consider the ordinary differential equation for a solution ${\displaystyle y(t)}$ evolving in time:

This ordinary differential equation (ODE) needs two initial conditions at, say, time ${\displaystyle t=0}$. Denote the initial conditions by ${\displaystyle y(0)=y_{0}}$ and ${\displaystyle dy/dt(0)=y'_{0}}$ where ${\displaystyle y_{0}}$ and ${\displaystyle y'_{0}}$ are some parameters. The following argument shows that the isochrons for this system are here the straight lines ${\displaystyle y_{0}+y'_{0}={\mbox{constant}}}$.

...
Wikipedia