The Collaborative International Dictionary of English v.0.48:

Detention \De*ten"tion\, n. [L. detentio: cf. F. d['e]tention.
   See Detain.]
   1. The act of detaining or keeping back; a withholding.
      [1913 Webster]

   2. The state of being detained (stopped or hindered); delay
      from necessity.
      [1913 Webster]

   3. Confinement; restraint; custody.
      [1913 Webster]

            The archduke Philip . . . found himself in a sort of
            honorable detention at Henry's court. --Hallam.
      [1913 Webster]

WordNet (r) 3.0 (2006):

detention
    n 1: a state of being confined (usually for a short time); "his
         detention was politically motivated"; "the prisoner is on
         hold"; "he is in the custody of police" [syn: detention,
         detainment, hold, custody]
    2: a punishment in which a student must stay at school after
       others have gone home; "the detention of tardy pupils"