• A Social impact bond, also known as Pay for Success Financing, a Pay for Success Bond or a Social Benefit Bond  or simply a Social Bond, is a contract with the public sector in which a commitment is made to pay for improved social outcomes that result in public sector savings
• The term was originally coined by Geoff Mulgan, Chief Executive of the Young Foundation
• The first Social Impact Bond was launched by UK-based Social Finance Ltd. in September 2010
• Social Impact Bonds are a type of bond, but not the most common type. While they operate over a fixed period of time, they do not offer a fixed rate of return
• Repayment to investors is contingent upon specified social outcomes being achieved
• Therefore, in terms of investment risk, Social impact bonds are more similar to that of a structured product or an equity investment.

Source;Investopedia