A bond with a coupon rate that is within 0.5\% of the current market rate. Current coupon bonds are typically less volatile than other bonds with lower coupons because the coupon rate is closer to that set by the market.
Because a current coupon bond is less volatile, it is also less likely to be called. It has implied call protection rather than an explicit call provision. Its inherent stability, however, also means that it won't offer as great of a return.