Volatility measures the degree of variation in a stock's price over time. High volatility means the price swings dramatically in either direction, while low volatility indicates more stable, predictable movements. The VIX index, often called the 'fear gauge,' measures expected volatility of the S&P 500 over the next 30 days based on options pricing. Volatility is not directional: it measures the magnitude of moves, not whether they're up or down. Market volatility tends to be highest during the first and last 30 minutes of regular trading hours, during major economic data releases (like jobs reports or Fed announcements), and during earnings season when many companies report results. Extended-hours trading (pre-market and after-hours) typically shows higher volatility per trade due to lower liquidity and wider spreads. Historical volatility looks at past price movements, while implied volatility is forward-looking and derived from options markets. Both metrics are essential for risk management and position sizing.