Quantitative easing
Large-scale purchases of Treasury and mortgage-backed securities by the central bank, which add reserves to the banking system.
When the federal funds rate is already at or near zero, the committee cannot lower it further to loosen policy. Quantitative easing is the tool used instead: the Fed buys longer-dated securities in size, which bids up their price and pushes down their yield.
The purchases are paid for with newly created reserves, so the Fed’s balance sheet grows on both sides — securities on the asset side, reserves on the liability side. The intent is to lower long-term borrowing costs directly, rather than waiting for a short-rate change to transmit outward.
QE describes the Fed's balance sheet expansion after 2008 and again in 2020. It is the counterpart to quantitative tightening.