Interest, etymologically a state or condition of being concerned in or having a share in anything, hence a legal or other claim to or share in property, benefits or advantages. Further developments of meaning are found in the application of the word to the benefits, advantages, matters of importance, &c., in which “interest” or concern can be felt, and to the feeling of concern so excited; hence also the word is used of the persons who have a concern in some common “interest,” e.g. the trading or commercial interest, and of the personal or other influence due to a connexion with specific “interests.” The word is derived from the Latin interesse (literally “to be between”), to make a difference, to concern, be of importance. The form which the word takes in English is a substantival use of the 3rd person singular of the present indicative of the Latin verb, and is due to a similar use in French of the older interest, modern intérêt. The earlier English word was interess, which survived till the end of the 17th century; the earliest example of “interest” in the New English Dictionary is from the Rolls of Parliament of 1450.
These meanings of “interest” are plainly derived from the ordinary uses of the Latin interesse. The origin of the application of the word to the compensation paid for the use of money or for the forbearance of a debt, with which, as far as present English law is concerned, this article deals, forms part of the history of Usury and Money-Lending (q.v.). By Roman law, where one party to a contract made default, the other could enforce, over and above the fulfilment of the agreement, compensation based on the difference (id quod interest) to the creditor’s position caused by the default of the debtor, which was technically known as mora, delay. This difference could be reckoned according as actual loss had accrued, and also on a calculation of the profit that might have been made had performance been carried out. Now this developed the canonist doctrine of damnum emergens and lucrum cessans respectively, which played a considerable part in the breaking down of the ecclesiastical prohibition of the taking of usury. The medieval lawyers used the phrase damna et interesse (in French dommages et intérêts) for such compensation by way of damages for the non-fulfilment of a contract, and for damages and indemnity generally. Thus interesse and intérêt came to be particularly applied to the charge for the use of money disguised by a legal fiction under the form of an indemnity for the failure to perform a contract.
At English common law an agreement to pay interest is not implied unless in the case of negotiable instruments, when it is supported by mercantile usage. As a general rule therefore debts certain, payable at a specified time, do not carry interest from that time unless there has been an express agreement that they should do so. But when it has been the constant practice of a trade or business to charge interest, or where as between the parties interest has been always charged and paid, a contract to pay interest is implied. It is now provided by the Civil Procedure Act 1833 that, “upon all debts or sums certain payable at a certain time or otherwise, the jury on the trial of any issue or in any inquisition of damages may if they shall think fit allow interest to the creditor at a rate not exceeding the current rate of interest, from the time when such debts or sums certain were payable, if such debts or sums be payable by virtue of some written instrument at a certain time; or if payable otherwise, then from the time when demand of payment shall have been made in writing, so as such demand shall give notice to the debtor that interest will be claimed from the date of such demand until the term of payment: provided that interest shall be payable in all cases in which it is now payable by law.” Compound interest requires to be supported by positive proof that it was agreed to by the parties; an established practice to account in this manner will be evidence of such an agreement. When interest is awarded by a court it is generally at the rate of 4%; under special circumstances 5% has been allowed.
