In The Money (ITM)
A call option with a strike price below the underlying price, or a put option with a strike price above the underlying price.
Explanation
ITM options have intrinsic value. The deeper ITM an option is, the higher its delta (closer to 1.0 for calls, -1.0 for puts) and the more it behaves like the underlying stock. ITM options have less time value relative to their total premium and are more expensive but have a higher probability of profit.
Example
AAPL trades at $155. The 150-strike call is $5 in the money. The 160-strike put is $5 in the money.