Vega
The rate of change of an option price with respect to a 1% change in implied volatility.
Explanation
Vega is highest for ATM, longer-dated options. Long options have positive vega (benefit from rising IV); short options have negative vega (benefit from falling IV). Vega is critical for earnings trades and volatility strategies. Unlike other Greeks, vega is not an actual Greek letter.
Example
A call with vega of 0.12 will gain $0.12 ($12 per contract) if implied volatility increases by 1 percentage point.