Historical volatility
The range of a price's swings measured on data that has already happened.
How to read the number
It is the reference point for implied volatility: the gap between the two shows whether the market prices future movement above or below the movement already seen.
When the metric lies
The answer depends on the window: a short one jumps after every violent day, a long one smooths away even a crisis. Two sources with different windows quote different numbers for one instrument.
Also known as: realised volatility