In mathematical finance, the Greeks are the quantities representing the sensitivities of the price of derivatives to a change in underlying parameters on which the value of an instrument is dependent. The name is used because the most common of these sensitivities are often denoted by Greek letters. Collectively these have also been called the risk sensitivities, risk measures or hedge parameters.
The Greeks are vital tools in risk management. Each Greek measures the sensitivity of the value of a portfolio to a small change in a given underlying parameter, so that component risks may be treated in isolation, and the portfolio rebalanced accordingly to achieve the desired exposure; see for example delta hedging.