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):

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

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSTALMENT, contracts. A part of a debt due by contract, and agreed to be
paid at a time different from that fixed for the, payment of the other part.
For example, if I engage to pay you one thousand dollars, in two payments,
one on the first clay of January, and the other on the first day of July,
each of these payments or obligations to pay will be an instalment .
     2. In such case each instalment is a separate debt so far that it may
be tendered at any time, or the first may be sued for although the other
shall not be due. Dane's Ab. vol. iii. ch. 93, art. 3, s. 11, page 493, 4; 1
Esp. R. 129; Id. 226; 3 Salk. 6, 18: Esp. R. 235; 1 Maule & Selw. 706.
     3. A debtor who by failing to pay three instalments of rent due on a
lease would forfeit his estate, may, in order to save it, tender one
instalment to prevent the forfeiture, although there may be two due at the
time, and he is not bound to tender both. 6 Toull. n. 688.