Sinking Fund

What Does Sinking Fund Mean?
A means of repaying funds that were borrowed through a bond issue. The issuer makes periodic payments to a trustee who retires part of the issue by purchasing the bonds in the open market.

Investopedia explains Sinking Fund
Rather than the issuer repaying the entire principal of a bond issue on the maturity date,  another company buys back a portion of the issue annually and usually at a fixed par value or at the current market value of the bonds, whichever is less. Should interest rates decline following a bond issue, sinking-fund provisions allow a firm to lessen the interest rate risk of its bonds as it essentially replaces a portion of existing debt with lower-yielding bonds.

From the investor's point of view, a sinking fund adds safety to a corporate bond issue: with it, the issuing company is less likely to default on the repayment of the remaining principal upon maturity since the amount of the final repayment is substantially less. This added safety affects the interest rate at which the company is able to offer bonds in the marketplace.