In finance (a), volatility (usually denoted by \(\sigma\)) is the degree of variation of a trading price series over time, usually measured by the Standard deviation of logarithmic returns (a).
Historic volatility measures a time series of past market prices. Implied volatility looks forward in time, being derived from the market price of a market-traded derivative (in particular, an option).
Bibliography
“Volatility (Finance).” 2022. Wikipedia, October. https://en.wikipedia.org/w/index.php?title=Volatility_(finance)&oldid=1116100450.