Saltpetre n. : Potassium nitrate; niter; a white crystalline substance, KNO3, having a cooling saline taste, obtained by leaching from certain soils in which it is produced by the process of nitrification (see Nitrification, 2). It is a strong oxidizer, is the chief constituent of gunpowder, and is also used as an antiseptic in curing meat, and in medicine as a diuretic, diaphoretic, and refrigerant.

Example:
How is Saltpetre used as a fertilizer?

Taxonomy
Potassium nitrate, saltpetre, nitrates, potassium, KNO3, wood ashes, g/100 mL, compounds, component, gunpowder, fertilizer, free encyclopedia, salts, nitrate of potash, nitrite, liquid, straw, 2007-10-19.

Defination from the web:
saltpetre (saltpeter in American English), from Medieval Latin sal petræ: "stone salt" or possibly "Salt of Petra" ... Urine has also been used in the manufacture of saltpetre for gunpowder. ...