caustic soda vs lye

caustic soda and lye both are nouns.

Word	Noun	Adjective	Verb	Adverb
caustic soda	Yes	No	No	No
lye	Yes	No	No	No

As nouns, lye is a hypernym of caustic soda; that is, lye is a word with a broader meaning than caustic soda:

caustic soda: a strongly alkaline caustic used in manufacturing soap and paper and aluminum and various sodium compounds
lye: a strong solution of sodium or potassium hydroxide

Other hypernyms of caustic soda include hydrated oxide, hydroxide.

caustic soda (noun)	lye (noun)
a strongly alkaline caustic used in manufacturing soap and paper and aluminum and various sodium compounds	a strong solution of sodium or potassium hydroxide