Strike price
The fixed price at which an option holder can buy (call) or sell (put) the underlying if the option is exercised.
Each listed option series has one strike, set by the exchange from a ladder of standard increments around the current share price. The strike decides whether an option is in, at or out of the money, and together with the premium it sets the break-even price.
Strikes close to the current price usually trade more actively, while far out-of-the-money strikes cost less per contract and are less likely to finish in the money.
Illustrative example (hypothetical numbers, not a real trade): with a stock at $200, a $210 call is out of the money by $10 and a $190 call is in the money by $10. At expiration with the stock at $215, the $210 call has $5.00 of intrinsic value, or $500 per contract.