installment (n.) Look up installment at
also instalment, 1580s, "induction into office, act of installing," from install + -ment.

The word meaning "a partial payment on account of debt due" (1776) earlier referred to the arrangement of payment (1732), and is 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.