sea biscuit vs hardtack

sea biscuit and hardtack both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea biscuit	Yes	No	No	No
hardtack	Yes	No	No	No

As nouns, sea biscuit and hardtack are synonyms defined as:

sea biscuit and hardtack: very hard unsalted biscuit or bread; a former ship's staple

Other synonyms of sea biscuit include pilot biscuit, pilot bread, ship biscuit.

sea biscuit (noun)	hardtack (noun)
very hard unsalted biscuit or bread; a former ship's staple	very hard unsalted biscuit or bread; a former ship's staple
	a mountain mahogany