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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" (see tardy). Sense of "educational slowness" is from 1907.
retardation re·tar·da·tion (rē'tär-dā'shən)
The condition of being relatively slow in mental, emotional, or physical development.
The extent to which something is held back or delayed.