An underlying index is the specific market index that serves as the reference point for index-based derivatives including futures, options, and ETFs. Popular underlying indices include S&P 500, NASDAQ-100, and VIX, which provide standardized benchmarks for broad market exposure and hedging strategies.

Underlying index methodology, including constituent selection, weighting schemes, and rebalancing procedures, directly affects derivative performance and tracking accuracy. Understanding index construction helps assess derivative behavior and potential tracking errors. Index derivatives enable efficient portfolio hedging and market exposure management.

Real-world example: E-mini S&P 500 futures use the S&P 500 index as the underlying, with futures prices closely tracking the index value and enabling traders to gain broad U.S. equity market exposure through a single instrument.