A contract that gives you the right to buy a specific stock at a fixed price (called the strike price) before a certain date. You are not required to buy the stock, only have the option to do so. You pay a fee upfront, called a premium, to buy this contract.
You buy a call option on Apple stock with a strike price of $150 that expires in one month, paying $5 per share. If Apple's stock rises to $160, you can exercise your right to buy it at $150, then immediately sell it at $160 for a $10 profit per share (minus the $5 premium you paid). If the stock stays below $150, you simply let the option expire and lose the $5 premium.
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