Greek fire or Byzantine fire was a weapon used by the Byzantine Empire. It was capable of discharging a stream of fluid which was ignited, and was very effective both on sea and land, but it was used primarily at sea. It is rumored that the key to Greek Fire's effectiveness was that it would continue burning under almost any conditions, even under water. Enemy ships were often afraid to come too near the Byzantine fleet because once within range the fire gave the Byzantines a strong advantage.
The secret of manufacturing the fluid was very carefully guarded, so well that today we still do not know how it was made.
Byzantine fire was largely responsible for many Byzantine military victories, and part of why the Empire managed to survive as long as it did, particularly near the end of the Empire when there were not enough inhabitatants of Byzantium to effectively defend it.