Volatility
How much an asset's price fluctuates over time — usually annualised standard deviation of returns.
Volatility measures how much an asset's price fluctuates over a period, typically expressed as the annualised standard deviation of daily returns. Higher volatility means a wider distribution of possible outcomes around the expected return.
Historical volatility is computed from realised past prices; implied volatility is backed out of option prices and represents the market's forward-looking estimate of future swings. The VIX is the most-watched implied-volatility index.
Volatility is symmetric — it captures upside surges as much as downside crashes. For pure downside-risk measures, see drawdown or value-at-risk. Volatility tends to cluster: calm follows calm, storms follow storms.