unhappiness vs sadness

unhappiness and sadness both are nouns.

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

As nouns, unhappiness and sadness are synonyms defined as:

unhappiness and sadness: emotions experienced when not in a state of well-being

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

unhappiness: state characterized by emotions ranging from mild discontentment to deep grief
sadness: the state of being sad

Other hyponyms of unhappiness include embitterment, sorrow, sorrowfulness.

unhappiness (noun)	sadness (noun)
emotions experienced when not in a state of well-being	the quality of excessive mournfulness and uncheerfulness
state characterized by emotions ranging from mild discontentment to deep grief	emotions experienced when not in a state of well-being
	the state of being sad