sadness vs feeling

sadness and feeling both are nouns.

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

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

sadness (noun)	feeling (noun)
the quality of excessive mournfulness and uncheerfulness	the experiencing of affective and emotional states
emotions experienced when not in a state of well-being	an intuitive understanding of something
the state of being sad	a physical sensation that you experience
	the sensation produced by pressure receptors in the skin
	a vague idea in which some confidence is placed
	the general atmosphere of a place or situation and the effect that it has on people