slowdown vs retardation

slowdown and retardation both are nouns.

Word	Noun	Adjective	Verb	Adverb
slowdown	Yes	No	No	No
retardation	Yes	No	No	No

As nouns, slowdown and retardation are synonyms defined as:

Other synonyms of slowdown include lag.

slowdown (noun)	retardation (noun)
the act of slowing down or falling behind	the act of slowing down or falling behind
	a decrease in rate of change
	lack of normal development of intellectual capacities
	any agent that retards or delays or hinders
	the extent to which something is delayed or held back