Greek fire, any of several flammable compositions that were used in warfare in ancient and medievaltimes. More specifically, the term refers to a mixture introduced by the Byzantine Greeks in the 7th century CE.

Greek Fire helped protect the besieged Byzantine Empire for many centuries. Its formula was a jealously guarded secret passed from Emperor to Emperor until the fall of the Empire in 1453.

Greek Fire would become the most potent weapon of Christendom for over 700 years. This would, in no small part, enable the Byzantines and Constantinople to resist its many enemies for as long as it did.

It was first used by the Greeks besieged in Constantinople (673–78). It ignited on contact with water, and was probably based on naphtha and quicklime. The Byzantines typically used it in naval battlesto great effect, as it could continue burning while floating on water. It provided a technological advantage and was responsible for many key Byzantine military victories, most notably the salvation of Constantinoplefrom two Arab sieges, thus securing the Empire's survival.

The impression made by Greek fire on the western European Crusaders was such that the name was applied to any sort of incendiary weapon, including those used by Arabs, the Chinese, and the Mongols. However, these were different mixtures and not the same formula as the Byzantine Greek fire, which was a closely guarded state secret. Byzantines also used pressurized nozzles or siphōns to project the liquid onto the enemy, in a manner resembling a modern flamethrower.

Although usage of the term "Greek fire" has been general in English and most other languages since the Crusades, original Byzantine sources called the substance a variety of names, such as "sea fire" (Medieval Greek: πῦρ θαλάσσιον pŷr thalássion), "Roman fire" (πῦρ ῥωμαϊκόν pŷr rhōmaïkón), "war fire" (πολεμικὸν πῦρ polemikòn pŷr), "liquid fire" (ὑγρὸν πῦρ hygròn pŷr), "sticky fire" (πῦρ κολλητικόν pŷr kollētikón) or "manufactured fire" (πῦρ σκευαστόν pŷr skeuastón).

The composition of Greek fire remains a matter of speculation and debate, with various proposals including combinations of pine resin, naphtha, quicklime, calcium phosphide, sulfur, or niter. In Titus Livy's history of Rome, priestesses of Bacchus are said to have dipped fire into the water, which did not extinguish, "for it was sulphur mixed with lime."

Source: Wikipedia - interestingengineering.com