lessening vs modification

lessening and modification both are nouns.

Word	Noun	Adjective	Verb	Adverb
lessening	Yes	No	No	No
modification	Yes	No	No	No

As nouns, modification is a hypernym of lessening; that is, modification is a word with a broader meaning than lessening:

lessening: a change downward
modification: an event that occurs when something passes from one state or phase to another

Other hypernyms of lessening include alteration, change.

lessening (noun)	modification (noun)
a change downward	the act of making something different (as e.g. the size of a garment)
	slightly modified copy; not an exact copy
	an event that occurs when something passes from one state or phase to another
	the grammatical relation that exists when a word qualifies the meaning of the phrase