sadness vs bereavement

sadness and bereavement both are nouns.

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

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

Other hyponyms of sadness include mourning, poignance, poignancy.

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