Index price
A weighted average of spot prices from several trading venues, used as the benchmark for derivatives.
The index price combines spot prices from a set of trading venues into one benchmark. Each venue's price is typically weighted by its liquidity or volume, and outliers or stale quotes are excluded. Derivatives exchanges use the index as the anchor for their mark price and funding calculations.
On our site, the index is the spot reference for perpetual and basis figures. Because it blends venues, it is less affected by one exchange's outage or a single erroneous trade.
Traders care because the index is what a derivative is really tracking. If the index is wrong or thin, every price built on it is off as well.
Worked example (illustrative numbers, not real data): three venues quote 50,000, 50,020 and 49,980 with equal weight, so the index is 50,000. If the third venue halts and is dropped, the index becomes 50,010, the average of the remaining two.