lessened vs diminished

lessened and diminished both are adjectives.

Word	Noun	Adjective	Verb	Adverb
lessened	No	Yes	No	No
diminished	No	Yes	No	No

As adjectives, lessened and diminished are synonyms defined as:

Other synonyms of lessened include vitiated, weakened.

lessened (adjective)	diminished (adjective)
impaired by diminution	(of an organ or body part) diminished in size or strength as a result of disease or injury or lack of use
decreased in severity; made less harsh	impaired by diminution
	made to seem smaller or less (especially in worth)
	(of musical intervals) reduction by a semitone of any perfect or minor musical interval