Etymology
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installment (n.2)

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

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installation (n.)
mid-15c., "action of installing," in reference to church offices or other positions, from Medieval Latin installationem (nominative installatio), noun of action from past participle stem of installare (see install). Of machinery, etc., "act of setting up, a placing in position for use," from 1882.
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install (v.)

also instal, formerly also enstall, early 15c., "place in (ecclesiastical) office by seating in an official stall," from Old French installer (14c.) or directly from Medieval Latin installare, from Latin in- "in" (from PIE root *en "in") + Medieval Latin stallum "stall," from a Germanic source (compare Old High German stal "standing place;" see stall (n.1)). Related: Installed; installing.

In the church of England the installation of a canon or prebendary of a cathedral consists in solemnly inducting him into his stall in the choir and his place in the chapter. [Century Dictionary]
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