'installment-based' is formed from the noun 'installment' (from 'install' + the suffix '-ment') combined with 'based' (past participle/adjective of 'base'). 'Install' originates from Middle French/Medieval Latin roots where 'in-' meant 'in' and the root related to placing or setting.

'install' developed from Medieval Latin 'installare' (to place in a stall or position), passed into Old/Middle French as 'installer', and entered English as 'install'. The noun 'installment' arose in Early Modern English as a noun meaning a portion of payment; 'installment-based' is a later compound adjective formed by joining the noun with 'based'.

Originally related to placing or setting something in position, the word family shifted in financial use so that 'installment' came to mean 'a portion of a payment'; 'installment-based' now describes arrangements or plans structured around such portions.