slow down vs retard

slow down is not a noun while retard is a noun.

Word	Noun	Adjective	Verb	Adverb
slow down	No	No	Yes	No
retard	Yes	No	Yes	No

As verbs, slow down and retard are synonyms defined as:

Other synonyms of slow down include decelerate, slow, slow up.

As verbs, retard is a hyponym of slow down; that is, retard is a word with a more specific, narrower meaning than slow down:

Other hyponyms of slow down include moderate, fishtail.

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

Other hypernyms of slow down include decelerate, slow, slow down, slow up.

slow down (noun)	retard (noun)
	a person of subnormal intelligence

slow down (verb)	retard (verb)
become less tense, rest, or take one's ease	lose velocity; move more slowly
lose velocity; move more slowly	cause to move more slowly or operate at a slower rate
become slow or slower	slow the growth or development of
reduce the speed of	be delayed
cause to proceed more slowly