retard vs diminish

retard is a noun but diminish is not a noun.

Word	Noun	Adjective	Verb	Adverb
retard	Yes	No	Yes	No
diminish	No	No	Yes	No

As verbs, diminish is a hypernym of retard; that is, diminish is a word with a broader meaning than retard:

Other hypernyms of retard include decrease, fall, lessen.

retard (noun)	diminish (noun)
a person of subnormal intelligence

retard (verb)	diminish (verb)
lose velocity; move more slowly	decrease in size, extent, or range
cause to move more slowly or operate at a slower rate	lessen the authority, dignity, or reputation of
slow the growth or development of
be delayed