Intrinsic value
The amount an option is in the money. For a call it is price minus strike, for a put it is strike minus price, and otherwise it is zero.
Intrinsic value is what an option would be worth if it were exercised immediately. It can never be negative, so out-of-the-money and at-the-money options have zero intrinsic value.
An option’s price is its intrinsic value plus its extrinsic value. At expiration only intrinsic value remains, which is why an in-the-money option at expiration trades close to the amount it is in the money.
Illustrative example (hypothetical numbers, not a real trade): with a stock at $52, a $45 call has $7.00 of intrinsic value, or $700 per contract, and a $55 put has $3.00, or $300 per contract. A $55 call on the same stock has zero intrinsic value, so its whole price is extrinsic value.