tedium vs dullness

tedium and dullness both are nouns.

Word	Noun	Adjective	Verb	Adverb
tedium	Yes	No	No	No
dullness	Yes	No	No	No

As nouns, dullness is a hypernym of tedium; that is, dullness is a word with a broader meaning than tedium:

tedium (noun)	dullness (noun)
dullness owing to length or slowness	lack of sensibility
the feeling of being bored by something tedious	without sharpness or clearness of edge or point
	the quality of lacking interestingness
	a lack of visual brightness
	the quality of being slow to understand