installment (n.1)

"a partial payment on account of debt due," 1776, earlier "the arrangement of payment" (1732), an alteration of Anglo-French estaler "to fix payments," from Old French estal "fixed position, place; stall of a stable, market, or choir," from a Germanic source akin to Old High German stal "standing place" (see stall (n.1)). General sense of "a part of a whole, furnished or produced in advance of the rest" is from 1823. Installment plan is from 1894. 

installment (n.2)

also instalment, 1580s, "induction into office, act of installing," from install + -ment.

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Definitions of installment from WordNet

installment (n.)
a payment of part of a debt; usually paid at regular intervals;
installment (n.)
a part of a broadcast serial;
Synonyms: episode / instalment
installment (n.)
a part of a published serial;
Synonyms: instalment
installment (n.)
the act of installing something (as equipment);
Synonyms: installation / installing / instalment