Greek fire

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An incendiary preparation first used by the Byzantine Greeks to set fire to enemy ships.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun historical A flammable substance first used by the Greeks of Constantinople to set fire to enemy ships, buildings etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mixture used by Byzantine Greeks that was often shot at adversaries; catches fire when wetted