At the money
An option whose strike is at or near the current underlying price. Nearly all of an at-the-money option's price is time value.
An at-the-money option has little or no intrinsic value, so its price is almost entirely extrinsic value. At-the-money options typically have deltas near 0.50 for calls and -0.50 for puts, and their gamma and theta are larger than those of contracts further from the money with the same expiration.
They are often used as the reference point for quoting implied volatility and for estimating an expected move.
Illustrative example (hypothetical numbers, not a real trade): with a stock at $150.20, the $150 call and the $150 put are both at the money. If the call trades at $4.10, only $0.20 of that is intrinsic value, and the other $3.90 is time value that will decline if the stock stays near $150 into expiration.