An option is in-the-money (ITM) when exercising it right now would be immediately favorable. For a Call option, this means the strike price is below the current market price. For a Put option, the direction flips — the strike price is above the current market price.
Related Reading
See the full ITM/ATM/OTM breakdown for both Calls and Puts in ITM, ATM, OTM Explained: Understanding Option Moneyness.
