Yeah, there's several definitions out there depending upon what you're doing with it or trading. My definition is a key change in supply/demand. Mark
Philosophically, it means uncertainty and fear. Mathematically, I'd define it as the amount of variance of the range of price.
I love trading in that environment. Can't wait for markets to return to uncertainty and fear for an extended period of time again. Soooooo ready.
There are all kinds of volatility... historical, implied, future, etc. But the really good stuff is when the general public hears bad news or reads their monthly statement and realizes that their 401K is turning into a 201K plan. Then they run for the door, blowing out everything, creating wonderful trading opportunities. It's even better when it spreads to Fool Service brokers and institutions. Love those panicked sellers!