What is Interest?

Legal Definition
Interest is payment from a borrower or deposit-taking financial institution to a lender or depositor of an amount above repayment of the principal sum (i.e. the amount borrowed). It is distinct from a fee which the borrower may pay the lender or some third party.

For example, a customer would usually pay interest to borrow from a bank, so they pay the bank an amount which is more than the amount they borrowed; or a customer may earn interest on their savings, and so they may withdraw more than they originally deposited. In the case of savings, the customer is the lender, and the bank plays the role of the borrower.

Interest differs from profit, in that interest is received by a lender, whereas profit is received by the owner of an asset, investment or enterprise. (Interest may be part or the whole of the profit on an investment, but the two concepts are distinct from one another from an accounting perspective.)

The rate of interest is equal to the interest amount paid or received over a particular period divided by the principal sum borrowed or lent.

Compound interest means that interest is earned on prior interest in addition to the principal. Due to compounding, the total amount of debt grows exponentially, and its mathematical study led to the discovery of the number e. In practice, interest is most often calculated on a daily, monthly, or yearly basis, and its impact is influenced greatly by its compounding rate.
-- Wikipedia
Legal Definition
Estates. The right which a man has in a chattel real, and more particularly in a future term. It is a word of less efficacy and extent than estates, though, in legal understanding, an interest extends to estates, rights and titles which a man has in or out of lands, so that by a grant of his whole interest in land, a reversion as well as the fee simple shall pass. Co. Litt. 345.
-- Bouviers Law Dictionary
Legal Definition
Contracts. The right of property which a man has in a thing, commonly called insurable interest. It is not easy to give all accurate definition of insurable interest. 1 Burr. 480; 1 Pet. R. 163; 12 Wend. 507 16 Wend. 385; 16 Pick. 397; 13 Mass. 61, 96; 3 Day, 108; 1 Wash. C. C. Rep. 409.

2. The policy of commerce and the various complicated. rights which different persons may have in the same thing, require that not only those who have an absolute property in ships and goods, but those also who have a qualified property therein, may be at liberty to insure them. For example, when a ship is mortgaged, after, the mortgage becomes absolute, the owner of the legal estate has an insurable interest, and the mortgagor, on account of his equity, has also an insurable interest. 2 T. R. 188 1 Burr. 489; 13 Mass. 96; 10 Pick. 40 and see 1 T. R. 745; Marsh. Ins. h. t.; 6 Meeson & Welshy, 224.

3. A man may not only insure his own life for the benefit of his heirs or creditors, and assign the benefit of this insurance to others having thus or otherwise an interest in his life, but be may insure the life of another in which he may be interested. Marsh. Ins. Index, h. t.; Park, Ins. Index, h. t.; 1 Bell's Com. 629, 5th ed.; 9 East, R. 72. Vide Insurance.
-- Bouviers Law Dictionary
Legal Definition
Evidence. The benefit which a person has in the matter about to be decided and which is in issue between the parties. By the term benefit is here understood some pecuniary or other advantage, which if obtained, would increase the, witness estate, or some loss, which would decrease it.

2. It is a general rule that a party who has an interest in the cause cannot be a witness. It will be proper to consider this matter by taking a brief view of the thing or subject in dispute, which is the object of the interest; the quantity of interest; the quality of interest; when an interested witness can be examined; when the interest must exist; how an interested witness can be rendered competent.

3. - 1. To be disqualified on the ground of interest, the witness must gain or lose by the event of the cause, or the verdict must be lawful evidence for or against him in another suit, or the record must be an instrument of evidence for or against him. 3 John. Cas. 83; 1 Phil. Ev. 36; Stark. Ev. pt. 4, p. 744. But an interest in the question does not disqualify the witness. 1 Caines, 171; 4 John. 302; 5 John. 255; 1 Serg. & R. 82, 36; 6 Binn. 266; 1 H. & M. 165, 168.

4. - 2. The magnitude of the interest is altogether immaterial, even a liability for the most trifling costs will be sufficient. 5 T. R. 174; 2 Vern. 317; 2 Greenl. 194; 11 John. 57.

5. - 3. With regard to the quality, the interest must be legal, as contradistinguished from mere prejudice or bias, arising from relationship, friendship, or any of the numerous motives by which a witness may be supposed to be influenced. Leach, 154; 2 St. Tr. 334, 891; 2 Hawk. ch. 46, s. 25. It must be a present, certain, vested interest, and not uncertain and contingent. Dougl. 134; 2 P. Wms. 287; 3 S. & R. 132; 4 Binn. 83; 2 Yeates, 200; 5 John. 256; 7 Mass. 25. And it must have been acquired without fraud. 3 Camp. 380; l M. & S. 9; 1 T. R. 37.

6. - 4. To the general rule that interest renders a witness incompetent, there are some exceptions. First. Although the witness may have an interest, yet if his interest is equally strong on the other side, and no more, the witness is reduced to a state of neutrality by an equipoise of interest, and the objection to his testimony ceases. 7 T. R. 480, 481, n.; 1 Bibb, R. 298; 2 Mass. R. 108; 2 S. & R. 119; 6 Penn. St. Rep. 322.

7. Secondly. In some instances the law admits the testimony of one interested, from the extreme necessity of the case; upon this ground the servant of a tradesman is admitted to prove the delivery of goods and the payment of money, without any release from the master. 4 T. R. 490; 2 Litt. R. 27.

8. - 5. The interest, to render the witness disqualified, must exist at the time of his examination. A deposition made at a time when the witness had no interest, may be read in evidence, although he has afterwards acquired an interest. 1 Hoff. R. 21.

9. - 6. The objection to incompetency on the ground of interest may be removed by an extinguishment of that interest by means of a release, executed either by the witness, when he would receive an advantage by his testimony, or by those who have a claim upon him when his testimony would be evidence of his liability. The objection may also be removed by payment. Stark. Ev. pt. 4, p. 757. See Benth. Rationale of Jud. Ev. 628-692, where he combats the established doctrines of the law, as to the exclusion on the ground of interest; and Balance.
-- Bouviers Law Dictionary
Legal Definition
In property. The most general term that can be employed to denote a property in lands or chattels. ' In its application to lands or things real, it is frequently used in connection with the terms "estate," "right," and "title," and, according to Lord Coke, it properly includes them all. Co.. Litt. 345b. See Ragsdale v. Mays, 65 Tex. 257; Hurst v. Hurst, 7 W. Va. 297; New York v. Stone, 20 Wend. (N. Y.) 142; State v. Mc-Kellop, 40 Mo. 185; Loventhal v. Home Ins. Co.., 112 Ala. 116, 20 South. 419, 33 L R. A. 258, 57 Am. St. Rep. 17. More particularly it means a right to have the advantage accruing from anything; any right in the nature of property, but less than title; a partial or undivided right; a title to a share. The terms "interest" and "title" are not synonymous. A mortgagor in possession, and a purchaser holding under a deed defectively executed, have, both of them, absolute as well as insurable interests in the property, though neither of them has the legal title. Hough v. City F. Ins. Co., 29 Co.nn. 20, 76 Am. Dec. 5Si.
See also
-- Black's Law Dictionary
Legal Definition
Any right, in the nature of property, less than title. See 73 Kan. 127, 117 Am. St. Rep. 460, 9 Ann. Cas. 459, 4 L. R. A. (N. S.) 654, 84 Pac. 717; compensation for the use of money. See 211 Mass. 171, Ann. Cas. 1913B, 206, 39 L. R. A. (N. S.) 120, 97 N. E. 1100.
-- Ballentine's Law Dictionary