Fig. 1.

Explanation of the isochrone. The closed curve represents the limit cycle orbit $C$. Curves on which all the points have the same phase are called isochrones, $I\left(\varphi \right)$. A crossing point of $C$ and $I\left(\varphi \right)$ is written as $X_0\left(\varphi \right)$. The centre of the limit cycle $C$, where all the isochrones meet, is a singular point where the phase cannot be defined.