Vega
An estimate of how much an option's price changes for a one-point change in implied volatility, holding other inputs constant.
Vega is quoted per one percentage point of implied volatility. It is positive for long calls and long puts, because higher implied volatility raises the value of both. It is larger for longer-dated options and for options near the money, so a change in IV moves those prices more.
Vega is the Greek behind IV crush. When implied volatility drops after an event, each long option on the underlying loses roughly its vega times the drop in its own implied volatility.
Illustrative example (hypothetical numbers, not a real trade): an option trades at $5.00 with a vega of 0.12 and implied volatility of 40%. If IV rises to 45% and nothing else changes, the option is worth about 5.00 + 5 × 0.12 = $5.60. If IV falls to 30%, it is worth about $3.80.