Index price
The index price is an average of a trading instrument's price on several spot exchanges, calculated by a derivatives exchange as a reference for its futures contracts.
Nobody trades at the index price; it is a calculated value. Futures exchanges use it, together with the contract’s own prices, to build the mark price that values positions and triggers liquidations, so that one spike on one market does not liquidate everyone. Each exchange picks its own sources and weights, and they change over time, so check your exchange’s documentation.
Learn more: where to find a good price chart.