Riba ( , ) is an Arabic word that 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 (reports describing the words, actions, or habits of the Islamic prophet Muhammad). While Muslims agree that riba is prohibited, not all agree on what precisely it is. It is often used as an Islamic term for interest charged on loans, and the belief this is based on — that there is a consensus among Muslims that all loan/bank interest is riba — forms the basis of a $2 trillion Islamic banking industry. However, not all scholars have equated riba with all forms of interest, or agree whether its use is a major sin or simply discouraged (makruh), or whether it is in violation of sharia (Islamic law) to be punished by humans rather than by Allah. There are two principal forms of riba. Most prevalent is the interest or other increase on a loan of cash, which is known as riba an-nasiya. Most Islamic jurists hold there is another type of riba, which is the simultaneous exchange of unequal quantities or qualities of a given commodity. This is known riba al-fadl. Etymology and definitions The word riba was used by the Arabs prior to Islam to refer to an "increase". In classical Islamic jurisprudence the definition of riba was "surplus value without counterpart." The difficulty of explaining what exactly riba means in Islam has been noted by early Islamic jurists such as Ibn Majah and Ibn Kathir, who quotes the second "Rashidun Caliph" Umar bin al-Khattab, “There are three things, If God's Messenger had explained them clearly, it would have been dearer to me than the world and what it contains: (These are) kalalah, riba, and khilafah.” Disagreeing is Muhammad Taqi Usmani — "one of the leading" modern day "religious experts on Islamic finance" — who argues that scripture concerning riba cannot be ambiguous (or mutashabihat) because God would not condemn a practice but leave its "correct nature" unknown to Muslims. Definitions Definitions of riba include: 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