The Collaborative International Dictionary of English v.0.48:

installment \in*stall"ment\, instalment \in*stal"ment\, n.
   1. The act of installing; installation.
      [1913 Webster]

            Take oaths from all kings and magistrates at their
            installment, to do impartial justice by law.
                                                  --Milton.
      [1913 Webster]

   2. The seat in which one is placed. [Obs.]
      [1913 Webster]

            The several chairs of order, look, you scour; . . .
            Each fair installment, coat, and several crest
            With loyal blazon, evermore be blest. --Shak.
      [1913 Webster]

   3. A portion of a debt, or sum of money, which is divided
      into portions that are made payable at different times;
      that portion of a debt payed back in any one payment; as,
      the next installment is due January first. Payment by
      installment is payment by parts at different times, the
      amounts and times being often definitely stipulated.
      --Bouvier.
      [1913 Webster +PJC]

   4. a part of a broadcast serial. [WordNet sense 1]

   Syn: episode.
        [WordNet 1.5]

   5. a part of a published serial. [WordNet sense 2]
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

installment
    n 1: a payment of part of a debt; usually paid at regular
         intervals
    2: a part of a broadcast serial [syn: episode, installment,
       instalment]
    3: a part of a published serial [syn: installment,
       instalment]
    4: the act of installing something (as equipment); "the
       telephone installation took only a few minutes" [syn:
       installation, installing, installment, instalment]