What is the meaning of retardation?

noun

• A decrease in rate of change.
  • usage: "the deceleration of the arms race"
  • synonyms: deceleration, slowing
• The extent to which something is delayed or held back.
• Any agent that retards or delays or hinders.
  • usage: "flame-retardant"
  • synonyms: retardant, retardent
• Lack of normal development of intellectual capacities.
  • synonyms: mental retardation, backwardness, slowness, subnormality
• The act of slowing down or falling behind.
  • synonyms: slowdown, lag