saltpeter vs saltpetre

saltpeter and saltpetre both are nouns.

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

As nouns, saltpeter and saltpetre are synonyms defined as:

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

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

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