lye vs caustic soda

lye and caustic soda both are nouns.

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

As nouns, caustic soda is a hyponym of lye; that is, caustic soda is a word with a more specific, narrower meaning than lye:

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

Other hyponyms of lye include caustic potash, potash, potassium hydroxide, sodium hydroxide.

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