At The Money (ATM)
An option whose strike price is equal to or very close to the current price of the underlying asset.
Explanation
ATM options have the highest extrinsic (time) value and are most sensitive to changes in implied volatility. They have a delta of approximately 0.50 for calls and -0.50 for puts, meaning they have roughly a 50% probability of expiring in the money.
Example
If AAPL trades at $150, the 150-strike call and 150-strike put are both at the money.