Volatility

The degree of variation in an asset price over time.

Volatility measures how dramatically an asset price fluctuates. High volatility means large price swings, with higher potential gains but also higher risk. Implied volatility is derived from options prices and reflects market expectations of future volatility. The VIX index measures S&P 500 implied volatility.

Volatility spiked after the surprise rate decision, sending the index up and down by hundreds of points.