Term: Volatility

Volatility is a way of describing how much and how fast a price moves around, in either direction, over some period of time. A stock that jumps 5% up and down within a day is volatile; a stock that creeps along in a tight quarter-point range all day is not.

Under the hood, volatility is usually calculated from the spread of past price changes (returns) around their average, using a statistical tool called standard deviation. A higher number means returns have been more scattered, i.e. bigger swings in both directions, not just down moves. This historical version is called "realized" or "historical" volatility, and it is distinct from "implied" volatility, which is a forward-looking estimate backed out of options prices and reflects what the market expects future movement to be, rather than what already happened.

The nuance that trips people up is that volatility is not the same as direction, and it is not the same as risk in the everyday sense of "losing money." A stock can be highly volatile while trending steadily upward, and a low-volatility stock can still grind you down slowly. Volatility measures the size and consistency of the swings, not whether they are good or bad for you. It also compounds with time in a non-obvious way: doubling your holding period does not double your expected range of movement, it roughly scales with the square root of time, which is why volatility figures are usually quoted on an annualized basis to make different timeframes comparable.

People also conflate volatility with liquidity. A thinly traded stock can look "volatile" simply because a small order moves the price a lot, which is really a liquidity problem wearing a volatility costume. True volatility is about the dispersion of prices over time regardless of why it happened.