A risk-management strategy that investors can use to guard against the loss of unrealized gains. The put option acts like an insurance policy - it costs money, which reduces the investor's potential gains from owning the security, but it also reduces his risk of losing money if the security declines in value.

If an investor purchased a stock for $10 that is now worth $20 but he has not sold it, he has unrealized gains of $10. If he doesn't want to sell the stock yet (perhaps because he thinks it will appreciate further) but he wants to make sure he doesn't lose the $10 in unrealized gains, he can purchase a put option for that same stock (called the "underlying stock") that will protect him for as long as the option contract is in force. If the stock continues to increase in price, say, going up to $30, the investor can benefit from the increase. If the stock declines from $20 to $15 or even to $1, the investor is able to limit his losses because of the protective put.