The security on which a derivative derives its value. For example, a call option on Google stock gives the holder the right, but not the obligation, to purchase Google stock at the price specified in the option contract. In this case, Google stock is the underlying security.
In derivative terminology, the underlying security is often referred to simply as "the underlying." An underlying security can be any asset, index, financial instrument or even another derivative. Generally, an underlying security's value should be independently observable by both parties, so that there is no potential for confusion regarding the value of the derivative. Investors dealing in derivatives must closely research the underlying security in order to ensure that they fully understand the factors affecting the value of the derivative.