Greek fire

Invented by Byzantines, Greek fire was a fearsome weapon. It had the ability to keep burning while on water. As such, it was often used in naval battles. The creation of it was a closely guarded secret and has now been lost. It was very similar to modern napalm, though it was weaker. Byzantine ships attached it to their fronts as a medival sort of flamethrower. After several accidents of the fire burning its own ship, it was no longer used.

A Byzantine ship using Greek fire in the late 11th century. Madrid Skylitzes manuscript.

The term "Greek fire" is normally used in English. However, most other languages use other terms such as "sea fire" (Greek: πῦρ θαλάσσιον), "Roman fire" (πῦρ ῥωμαϊκόν), "war fire" (πολεμικὸν πῦρ), "liquid fire" (ὑγρὸν πῦρ), or "processed fire" (πῦρ σκευαστόν).