The use of an option or other hedging instrument in order to limit or reduce losses in the case of a decline in the value of the underlying security. Downside protection often involves the purchase of an option to hedge a long position. Other methods of downside protection include using stop losses or purchasing assets that are negatively correlated to the asset you are trying to hedge.
An example of downside protection would be the purchase of a put option for a particular stock. If an investor already owns shares and the price of that stock falls, the value of the option will increase and thus limit the total loss exposure.