The specific price at which you have the right to buy (for a call option) or sell (for a put option) a stock. This price is set when you buy the option and does not change, even if the stock price moves significantly.
You purchase a call option with a strike price of $100 per share. Even if the stock price jumps to $120, your strike price remains $100, meaning you can still buy at that lower price. The strike price is what makes the option valuable or worthless depending on where the stock actually trades.
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