The Collaborative International Dictionary of English v.0.48:

Retard \Re*tard"\, v. t. [imp. & p. p. Retarded; p. pr. & vb.
   n. Retarding.] [L. retardare, retardatum; pref. re- re- +
   tardare to make slow, to delay, fr. tardus slow: cf. F.
   retarder. See Tardy.]
   1. To keep delaying; to continue to hinder; to prevent from
      progress; to render more slow in progress; to impede; to
      hinder; as, to retard the march of an army; to retard the
      motion of a ship; -- opposed to accelerate.
      [1913 Webster]

   2. To put off; to postpone; as, to retard the attacks of old
      age; to retard a rupture between nations.
      [1913 Webster]

   Syn: To impede; hinder; obstruct; detain; delay;
        procrastinate; postpone; defer.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Retarded \Re*tard"ed\, n.
   having a limited or below normal mental ability; same as

   mentally retarded; -- used especially in relation to
      performance in academic tasks.
      [PJC]

WordNet (r) 3.0 (2006):

retarded
    adj 1: relatively slow in mental or emotional or physical
           development; "providing a secure and sometimes happy life
           for the retarded" [ant: precocious]
    n 1: people collectively who are mentally retarded; "he started
         a school for the retarded" [syn: mentally retarded,
         retarded, developmentally challenged]