severity vs seriousness

severity and seriousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
severity	Yes	No	No	No
seriousness	Yes	No	No	No

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

Other hyponyms of severity include foulness, raininess, distressfulness.

severity (noun)	seriousness (noun)
excessive sternness	the trait of being serious
used of the degree of something undesirable e.g. pain or weather	the quality of arousing fear or distress
something hard to endure	an earnest and sincere feeling
extreme plainness