bereavement vs sadness

bereavement and sadness both are nouns.

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

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

Other hypernyms of bereavement include sorrow, sorrowfulness.

bereavement (noun)	sadness (noun)
state of sorrow over the death or departure of a loved one	the quality of excessive mournfulness and uncheerfulness
	emotions experienced when not in a state of well-being
	the state of being sad