1. A line (imaginary or on a map) connecting points at which a particular event occurs or occurred at the same time: The isochron ratio chart showed a group of rocks and mineral samples that had been formed at the same time.
2. A line (imaginary or on a map) connecting points on the sea-floor formed at the same time: In class the students were told to prepare a isochron of a cross section of the ocean crust with linking points of the same age.