Sterilisation refers to the actions taken by a central bank to offset the impact of its foreign exchange operations on the domestic money supply. 

When a central bank intervenes in the foreign exchange market by buying or selling foreign currencies, it affects the domestic money supply. 

Open Market Operations (OMO) is one of the primary tools used by central banks, including the Reserve Bank of India (RBI), to conduct monetary policy. 

In OMO, the central bank buys or sells government securities (bonds) in the open market to influence the liquidity in the economy. When the RBI conducts OMO, it impacts the money supply in the economy. If the RBI buys government securities, it injects money into the system, increasing the money supply. To prevent this injection of money from creating inflationary pressures, the RBI engages in sterilisation. 

Sterilisation involves the simultaneous sale or purchase of other securities, typically treasury bills, to offset the impact of the initial open market operation.