early 15c., "fact or action of making slower in movement or time," from Latin retardationem (nominative retardatio) "a delaying," noun of action from past participle stem of retardare "to make slow, delay, keep back, hinder," from re- (see re-), + tardare "to slow," related to tardus "slow, sluggish" (see tardy). Sense of "educational slowness" is from 1907.