Vega

Vega measures how much an option's price changes when implied volatility moves one percentage point. An option with a vega of 0.12 gains $0.12 per share ($12 per contract) if IV rises from 40% to 41%, and loses the same if IV drops a point.

Long options are long vega, short options are short vega — a fact that matters most around events. Buying a straddle before earnings is as much a bet on IV as on price, and a post-event IV collapse can erase gains from a correct directional call. Vega is largest for at-the-money options with more time to expiration.