Index Arbitrage

What Does Index Arbitrage Mean?
An investment strategy that attempts to profit from the differences between actual and theoretical futures prices of the same stock index. This is done by simultaneously buying (or selling) a stock index future while selling (or buying) the stocks in that index.

Investopedia explains Index Arbitrage
This is done with program trading. Using software that monitors both a stock index and futures contracts on the index, traders can be notified when there is a larger than normal gap.