Strike Price
The fixed price at which the option holder can buy (call) or sell (put) the underlying asset if they choose to exercise.
The strike price (also called the exercise price) is written into the contract at inception and does not change. For a call, it is the price the buyer would pay for the underlying. For a put, it is the price the buyer would receive.
The relationship between the strike and the current spot price determines whether the option is in the money, at the money, or out of the money — which directly affects the premium and the delta.
On the desk
A trader buys an SPY $530 call when SPY is at $525. The $530 strike is $5 out of the money. SPY must clear $530 before the call has intrinsic value; the trader needs it to reach at least $530 + premium to break even.