installment n.1

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]

common suffix of Latin origin forming nouns, originally from French and representing Latin -mentum, which was added to verb stems to make nouns indicating the result or product of the action of the verb or the means or instrument of the action. In Vulgar Latin and Old French it came to be used as a formative in nouns of action. French inserts an -e- between the verbal root and the suffix (as in commenc-e-ment from commenc-er; with verbs in ir, -i- is inserted instead (as in sent-i-ment from sentir).

Used with English verb stems from 16c. (for example amazement, betterment, merriment, the last of which also illustrates the habit of turning -y to -i- before this suffix).

The stems to which -ment is normally appended are those of verbs; freaks like oddment & funniment should not be made a precedent of; they are themselves due to misconception of merriment, which is not from the adjective, but from an obsolete verb merry to rejoice. [Fowler]