sadness vs loneliness

sadness and loneliness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sadness	Yes	No	No	No
loneliness	Yes	No	No	No

As nouns, loneliness is a hyponym of sadness; that is, loneliness is a word with a more specific, narrower meaning than sadness:

sadness (noun)	loneliness (noun)
the quality of excessive mournfulness and uncheerfulness	a disposition toward being alone
emotions experienced when not in a state of well-being	sadness resulting from being forsaken or abandoned
the state of being sad	the state of being alone in solitary isolation