Volatility is a statistical measure of the amount by which a security price oscillates, usually above and below its mean, without regard to its trend, over a specified period. This measure often referrs to the rate of change in price, expressed as a percentage and computed as the standard deviation of percentage change in price. Like price, volatility is mean reverting, which means that there is a tendency for it to revert back to its mean. This is not an absolute, though. High volatility values are associated with extreme sentiment and high risk.
Practical volatility measures are:
The change in price over n days.
The maximum price fluctuation (range) during the n days.
The average true range over the past n days.
The sum of the absolute price changes over n days.