Roll-down
The price gain that comes from a bond ageing and moving onto a shorter — and therefore lower — section of the yield curve.
How to read the number
With the curve standing still, the return combines the coupon and this slide, and the steeper the section the bond sits on, the larger the slide.
When the metric lies
The effect exists only while the curve is unchanged. A shift upward in rates wipes it out at once, so treating it as a guaranteed part of the return is a mistake.
Also known as: rolling down the curve, curve roll