Why it matters
Duration is what turns an abstract move in yields into a concrete gain or loss. Two bonds can face the identical change in market conditions and one barely moves while the other reprices severely; the difference is duration. Without it, “yields rose” carries no information about consequences.
Arithmetic, not measurement: price change is approximately duration multiplied by the change in yield. A first-order estimate that overstates losses and understates gains on large moves.
It also explains a pattern that recurs far outside bond markets. The longer you have to wait for money, the more its present value depends on the rate used to discount it. That applies to a thirty-year government bond and equally to a company whose profits are expected a decade from now. When the phrase “long-duration assets” appears in commentary about equities, this is what it means: not that anyone bought a bond, but that the valuation rests on distant cash flows and therefore carries bond-like sensitivity to rates.
For anyone holding debt, duration is the variable that converts a policy environment into a balance sheet outcome. Institutions do not usually get into difficulty because a borrower defaulted; they get into difficulty because rates moved and the assets they held had more duration than their funding did.
What to watch for
Duration is not maturity. A thirty-year bond has a maturity of thirty years and a duration meaningfully shorter, because the coupon payments arrive along the way and pull the average timing of the cash flows forward. A bond that pays nothing until redemption has a duration equal to its maturity. Longer maturity generally means longer duration, but the two numbers are not interchangeable.
It is a local approximation, not a constant. Duration describes the price response to a small change in yields. For large moves the relationship curves, and the estimate understates gains and overstates losses. It also changes as the bond ages and as yields themselves move.
The word does two jobs. In fixed income it is a measured quantity. Applied to equities — “long-duration growth stocks” — it is an analogy about valuation sensitivity, with no calculated figure behind it. Both usages are legitimate; treating a claim about the second as if it had the precision of the first is not.
Reported duration figures for bond indices and funds are published by the index providers and in fund documentation, where they are stated as effective or modified duration rather than left implicit.