noun (n)

• a decrease in rate of change(noun.attribute)

  Synonym:
  deceleration, slowing
  Antonym:
  acceleration
  

  source: wordnet30
• the extent to which something is delayed or held back(noun.time)

  source: wordnet30
• any agent that retards or delays or hinders(noun.object)

  Synonym:
  retardant, retardent
  

  source: wordnet30
• lack of normal development of intellectual capacities(noun.cognition)

  Synonym:
  backwardness, mental retardation, slowness, subnormality
  

  source: wordnet30
• the act of slowing down or falling behind(noun.act)

  Synonym:
  lag, slowdown
  

  source: wordnet30
• The act of retarding; hindrance; the act of delaying; as, the retardation of the motion of a ship; -- opposed to acceleration.(noun)

  source: webster1913