dullness vs tedium

dullness and tedium both are nouns.

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

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

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