loneliness vs sadness

loneliness and sadness both are nouns.

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

As nouns, sadness is a hypernym of loneliness; that is, sadness is a word with a broader meaning than loneliness:

Other hypernyms of loneliness include unhappiness.

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