the.com/installment

means One of several smaller payments (or chunks of a larger thing) that together make up the whole, spread out over time instead of all at once.

from Comes from Anglo-French estalement, 'a fixing or settling of payment,' rooted in the verb estaler, related to stall — the same family that gives us a market stall, a fixed place or position. The idea was payments 'fixed' or set at agreed points. The extra 'n' crept into English spelling later, likely by confused association with 'install' (to set in place), which is why the word looks like it has nothing to do with money until you trace it back.