Riba is the unjustified increment in borrowing or lending money, paid in kind or in money above the amount of loan, as a condition imposed by the lender or voluntarily by the borrower. Riba defined in this way is called in fiqh riba al-duyun (debt usury). (Abdel-Rahman Yousri Ahmad).
Riba (Arabic: ربا ,الربا، الربٰوة‎ ) can be roughly translated as "usury", or unjust, exploitative gains made in trade or business under Islamic law. Riba is mentioned and condemned in several different verses in the Qur'an (3:130, 4:161, 30:39 and perhaps most commonly in 2:275-2:280). It is also mentioned in many hadith.