Riba is a concept in Islamic banking that refers to charged interest. It is forbidden under Sharia, Islamic religious law, because it is thought to be exploitive. Depending on the interpretation, riba may only refer to excessive interest; however to others, the whole concept of interest is riba, and thus is unlawful.

While Muslims agree that riba is prohibited, not all agree on what it is, or whether it should be punished by humans. It is often used as an Islamic term for interest charged on loans, and this belief—that there is a consensus among Muslims that interest is riba—is the basis of a $2 trillion Islamic banking industry. However, not all scholars equated riba with all forms of interest, and among those who do there is also disagreement over whether it is a major sin and against sharia (Islamic law), or simply discouraged (makruh).
In addition to the unjust gains made from repayment of a loan—the full name of which is riba an-nasiya—most Islamic jurists believe there is another type of riba, riba al-fadl: the simultaneous exchange of unequal quantities or qualities of a given commodity.
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