slowing vs retardation

slowing and retardation both are nouns.

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

As nouns, slowing and retardation are synonyms defined as:

Other synonyms of slowing include deceleration.

slowing (noun)	retardation (noun)
a decrease in rate of change	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