Volatility

Volatility is the degree and speed of price fluctuation in a security or market, most commonly measured statistically as the standard deviation of returns over a given period — a way of quantifying how much a price bounces around, in either direction.

Two stocks can deliver the identical return over a year and have completely different volatility along the way. One might climb steadily with small day-to-day moves; the other might swing wildly up and down before arriving at the same endpoint. The first is low-volatility, the second high-volatility, even though an investor who bought and held either one for the full year ended up in exactly the same place.

Volatility isn't the same as risk

The two get used interchangeably, but they're not identical. Volatility describes the size and frequency of price swings. Risk, in the sense that matters most to an investor, is closer to the chance of a permanent loss of capital — money that doesn't come back. A stock that drops sharply and recovers fully was volatile, but the investor who held through it didn't actually lose anything permanently. A stock that declines and never recovers because the underlying business deteriorated is a different, more serious kind of risk, even if its price path along the way looked calmer.

Why it matters anyway

Even setting aside that distinction, volatility matters in practice because it's what tests an investor's temperament and time horizon. Sharp swings that are perfectly survivable for money you won't need for twenty years can force a badly timed, permanent loss if that same money is needed in six months and has to be sold at a low point.