Definition of 'Prepayment Risk'
The risk associated with the early unscheduled return of principal on a fixed-income security. Some fixed-income securities, such as mortgage-backed securities, have embedded call options which may be exercised by the issuer, or in the case of a mortgage-backed security, the borrower.
The yield-to-maturity of such securities cannot be known for certain at the time of purchase since the cash flows are not known. When principal is returned early, future interest payments will not be paid on that part of the principal. If the bond was purchased at a premium (a price greater than 100) the bond's yield will be less than what was estimated at the time of purchase.
Investopedia explains 'Prepayment Risk'
For a bond with an embedded call option, the higher a bond's interest rate relative to current interest rates, the higher the prepayment risk.
For example, on a mortgage-backed security, the higher the interest rate relative to current interest rates, the higher the probability that the underlying mortgages will be refinanced. Investors who pay a premium for a callable bond with a high interest rate take on prepayment risk. In addition to being highly correlated with falling interest rates, mortgage prepayments are highly correlated with rising home values, as rising home values provide incentive for borrowers to trade up in homes or use cash-out refinances, both leading to mortgage prepayments.