A protective put means buying a Put option on a stock you already own, as a hedge against a potential price decline. If the stock falls, the Put gains value, offsetting some of the loss on the shares — while letting you keep the shares themselves and any long-term upside potential, at the cost of the premium paid, similar to an insurance premium.
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See the full strategy with a worked example and cost trade-off in Protective Put Strategy: How to Insure Your Stock Portfolio.