Isophase light
An isophase light is a rhythmic light in which all durations of light and darkness are equal. It is normally used in light signals in lighthouses. Different lighthouses have different frequencies and not all are isophase and mariners can find out which light house they are near by the characteristic of the light signal.
The prefix derives from the Greek iso- meaning "same".
Other types of rhythmic lights used by lighted aids to navigation are flashing lights and occulting lights.
References
- Glossary of terms in any USCG Lights List