Zero lower bound (“ZLB”) is when the short-term nominal interest rate is at or near zero, causing a liquidity trap and limiting the capacity that the central bank has to stimulate economic growth.

Zero-bound refers to the lowest level that interest rates can fall to, and logic dictates that zero would be that level. The main arrow in a central bank's monetary policy quiver is interest rates. The bank will manipulate interest rates to either stimulate a stagnating economy or dampen an overheating one. Clearly, there are limits, especially at the lower end of the range.

The zero-bound is the lower limit that rates can be cut to, but no further. When this level is reached, and the economy is still underperforming, then the central bank can no longer provide stimulus via interest rates. Economists use the term liquidity trap to describe this scenario.

When faced with a liquidity trap, alternative procedures for monetary stimulus often become necessary. Conventional wisdom was that interest rates could not move into negative territory, meaning once interest rates reach zero or are close to zero, for example, 0.01%, monetary policy has to be altered to continue to stabilize or stimulate the economy.