diminished vs lessened

diminished and lessened both are adjectives.

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

As adjectives, diminished and lessened are synonyms defined as:

Other synonyms of diminished include vitiated, weakened.

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