Definitions for "underlying security"
The security on which options are being bought or sold.
The physical security upon which a synthetic security is based. For example, CBA stock is the underlying security to CBA options.
The security upon which a derivative (option, index, futures contract) or other security is based. Eg, an ASA option is based upon the price of ASA stock. The S&P 500 futures index is based upon the S&P 500 stock index.
Keywords:  see
see underlier.