From Longman Dictionary of Contemporary Englishsimple interestˌsimple ˈinterest noun [uncountable] BFLinterest that is calculated on the sum of money that you first invested, and does not include the interest it has already earned → compound interest
Examples from the Corpus
simple interest• She argued that BofA underpaid because it used only simple interest in refunding the overcharged fees.From Longman Business Dictionarysimple interestˌsimple ˈinterest [uncountable]FINANCE interest calculated only on the PRINCIPAL (=the original amount invested), not on any accrued interestI charged him 5% simple interest on the principal per week. → interest 