In economics, stimulus refers to attempts to use "monetary or "fiscal policy (or "stabilization policy in general) to stimulate the economy. Stimulus can also refer to monetary policies like lowering interest rates and "quantitative easing.
A stimulus is sometimes colloquially referred to as "priming the pump" or "pump priming", In the 1930s, "President Herbert Hoover was accused of "pump priming", and "President Franklin D. Roosevelt used the term favorably.
Often, the underlying assumption is that, due to a "recession, production and "employment are far below their sustainable potential (see "NAIRU) due to lack of "demand. It is hoped that this will be corrected by increasing demand and that any adverse side effects from stimulus will be mild.
Fiscal stimulus refers to increasing government consumption or transfers or lowering taxes. Effectively this means increasing the rate of growth of "public debt, except that particularly "Keynesians often assume that the stimulus will cause sufficient "economic growth to fill that gap partially or completely. See "multiplier (economics).
Monetary stimulus refers to lowering interest rates, quantitative easing, or other ways of increasing the amount of money or credit.
For example, "Milton Friedman argued that the "Great Depression was caused by the fact that the "Federal Reserve did not counteract the sudden reduction of money stock and velocity. "Ben Bernanke argued, instead, that the problem was lack of "credit, not lack of money, and hence, during the "financial crisis, the Federal Reserve led by Bernanke provided additional credit, not additional liquidity (money), to stimulate the economy back on trail. Jeff Hummel has analyzed the different implications of these two conflicting explanations. President of the Federal Reserve Bank of Richmond, Jeffrey Lacker, with Renee Haltom, has criticized Bernanke's solution because "it encourages excessive risk-taking and contributes to financial instability." 
It is often argued that fiscal stimulus typically increases "inflation, and hence must be counteracted by a typical central bank. Hence only monetary stimulus could work. Counter-arguments say that if the production gap is high enough, the risk of inflation is low, or that in depressions inflation is too low but central banks are not able to achieve the required inflation rate without fiscal stimulus by the government.
Monetary stimulus is often considered more neutral: decreasing interest rates make additional "investments profitable, but yet only the most additional investments, whereas fiscal stimulus where the government decides the investments may "lead to "populism or "corruption. On the other hand, the government can also take "externalities into account, such as how new roads or railways benefit users that do not pay for them, and choose investments that are even more beneficial although not profitable.
Hoover and his secretary of the treasury, Andrew Mellon (himself often wrongly portrayed as a defender of laissez faire), proposed $400 million in new federal buildings as well as $175 million in public works through the federal Shipping Board. These proposals met with approval from the professoriate as examples of 'constructive industrial statesmanship.' They were also ridiculed as 'pump priming' in a New York Tribune editorial cartoon of April 8, 1930...