saltpetre vs saltpeter

saltpetre and saltpeter both are nouns.

Word	Noun	Adjective	Verb	Adverb
saltpetre	Yes	No	No	No
saltpeter	Yes	No	No	No

As nouns, saltpetre and saltpeter are synonyms defined as:

saltpetre and saltpeter: (KNO3) used especially as a fertilizer and explosive

Other synonyms of saltpetre include niter, nitre, potassium nitrate.

saltpetre (noun)	saltpeter (noun)
(KNO3) used especially as a fertilizer and explosive	(KNO3) used especially as a fertilizer and explosive