VIX
CBOE’s index of 30-day implied volatility on the S&P 500, computed from a strip of SPX option prices across strikes. Quoted in annualized percentage points, it is the market’s standard gauge of near-term expected equity volatility.
The VIX is not tradable itself; futures and options on it are, and they price where the index is expected to be at their expiration — which is why VIX products can move very differently from the spot index.
Rules of thumb: VIX/√12 approximates the implied 30-day move (VIX 20 ≈ 5.8% monthly). Its long-run average sits in the high teens, but it spends most of its life below that and occasional episodes far above it.
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