white phosphorus

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

• noun A highly unstable, waxy white or yellowish allotrope of phosphorus, P₄, that spontaneously ignites upon exposure to oxygen, used especially in bombs and artillery shells as an incendiary agent or to produce a smoke screen.

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

• noun chemistry The most common allotrope of phosphorus; a yellow-white waxy solid that is insoluble in water and spontaneously combusts on exposure to air to form clouds of phosphorus pentoxide; used as a military smokescreen and as an incendiary weapon.