The noun volatility is the characteristic of changing often and unpredictably. Your sister's volatility might be shown in how quickly she switches from laughing to crying. If you talk about the volatility of the stock market, stock prices are most likely fluctuating wildly. In chemistry, volatility means the speed with which a substance changes from solid to liquid, liquid to vapor, and so on. The root is the Latin volatilis, "fleeting, transitory, flying."