Usury is the practice of making loans that are seen as unfairly enriching the lender. The term may be used in a moral sense—condemning taking advantage of others' misfortunes—or in a legal sense, where an interest rate is charged in excess of the maximum rate that is allowed by law. A loan may be considered usurious because of excessive or abusive interest rates or other factors defined by the laws of a state. Someone who practices usury can be called a usurer, but in modern colloquial English may be called a loan shark. In many historical societies including ancient Christian, Jewish, and Islamic societies, usury meant the charging of interest of any kind, and was considered wrong, or was made illegal. During the Sutra period in India there were laws prohibiting the highest castes from practicing usury. Similar condemnations are found in religious texts from Buddhism, Judaism, Christianity, and Islam. At times, many states from ancient Greece to ancient Rome have outlawed loans with any interest. Though the Roman Empire eventually allowed loans with carefully restricted interest rates, the Catholic Church in medieval Europe, as well as the Reformed Churches, regarded the charging of interest at any rate as sinful. Religious prohibitions on usury are predicated upon the belief that charging interest on a loan is a sin.

सूदखोरी वो व्यवस्था होती है जिसमें कर्ज़ अत्यधिक ब्याज पर या कोई अनैतिक तरीके से दिया जाता है। इस व्यवस्था में कर्ज़ लेने वाले का लाभ नहीं बल्कि कर्ज देने वाला का लाभ प्रधान होता है। ऐतिहासिक तौर पर और आज भी कई समाज में कर्ज पर ब्याज लेना ही सूदखोरी माना जाता था। ईसाई धर्म में ब्याज लेना गलत आचरण माना जाता था। इस कारण ईसाई समाजों में यहूदी ही कर्ज मुहैया कराते थे। प्राचीन हिन्दू ग्रंथों में भी सूदखोरी को गलत माना गया है।