Define high-interest. high-interest synonyms, high-interest pronunciation, high- interest translation, English dictionary definition of high-interest. adj to be repaid at a high rate of interest ThesaurusAntonymsRelated WordsSynonymsLegend:. An interest rate is the amount of interest due per period, as a proportion of the amount lent, Other interest rates apply over different periods, such as a month or a day, but they are usually annualised. "since modern capital markets came into existence, there have never been such high long-term rates" as in this period . Interest rate is a money term you need to understand. Credit cards, car loans, personal loans and other types of loans usually have higher interest rates.