Which of the following is NOT one of the option greeks?

The term "option greeks" refers to various measures that help traders understand the risks associated with options and how options prices may change with different variables. These greeks provide insights into how sensitive the price of an option is to changes in the underlying asset's price, time decay, volatility, and interest rates.

Delta measures the rate of change of the option's price concerning changes in the underlying asset’s price. Gamma quantifies the rate of change of delta itself, indicating how much the delta would change if the underlying price moves. Vega measures the sensitivity of the option's price to changes in the volatility of the underlying asset, highlighting how the option price might increase or decrease as volatility fluctuates.

In contrast, the term "sigma" is not recognized as one of the standard option greeks. Instead, sigma is often used in finance to refer to standard deviation or volatility in a broader context, but it does not represent a specific measure used in the analysis of options pricing. This distinction makes it clear why sigma is not included in the group of option greeks.