index can refer to various things depending on the context:

1. In financial markets, it is a statistical measure representing a portfolio of securities.
2. In databases, it is a data structure improving data retrieval efficiency.
3. In books, it is a list of terms with page references.
4. In mathematics, it can denote the power to which a number is raised.
5. In general, it refers to a sign, indicator, or measure of something.