Merriam-Webster's Collegiate® Dictionary, 11th Edition
INSTALLMENT
I. noun also instalment
Date: 1589
installation 1,
II. noun also instalment
Etymology: alteration of earlier estallment payment by installment, from estall to arrange payments by installment, from Anglo-French estaler, from estal station, stall, division of a stage, of Germanic origin; akin to Old High German stal place, stall
Date: 1776
one of the parts into which a debt is divided when payment is made at intervals,
2. one of several parts (as of a publication) presented at intervals, one part of a serial story, installment adjective