The strike price on an index option. An index option is a call or put option contract in which the underlying asset is an index of any sort. For example, in a call, an investor may buy the right to an index on or before the expiration date at a certain strike index. Obviously, one cannot buy or sell a physical index; so the underlying asset is said to be the dollar value of an index at a certain date and time multiplied by $100. Because physical delivery is not possible, when a stock index option is exercised, the delivery is the cash value of the strike index.