In finance, the underlying of a derivative is an asset, basket of assets, index, or even another derivative, such that the cash flows of the (former) derivative depend on the value of this underlying. There must be an independent way to observe this value to avoid conflicts of interest.
According to the Financial Accounting Standards Board (FASB)'s Statement of Financial Accounting Standards No. 133 (FAS 133) - Accounting for Derivative Instruments and Hedging Activities, an underlying is a specified interest rate, security price, commodity price, foreign exchange rate, index of prices or rates, or other variable (including the occurrence or nonoccurrence of a specified event such as a scheduled payment under a contract). An underlying may be a price or rate of an asset or liability but is not the asset or liability.