The measurement of an option's sensitivity to changes in the volatility of the underlying asset. Vega represents the amount that an option contract's price changes in reaction to a 1% change in the volatility of the underlying asset. Volatility measures the amount and speed at which price moves up and down, and is often based on changes in recent, historical prices in a trading instrument. Vega changes when there are large price movements (increased volatility) in the underlying asset, and falls as the option approaches expiration. Vega is one of a group of Greeks used in options analysis, and is the only one not represented by a Greek letter.