In its simplest form, indexing is the process of scanning the text of multiple electronic documents to build a database table of search terms that correspond to those documents. It's what Google does for Internet search, and it's also a key component in e-discovery. Indexing enables optimized search and retrieval, and is particularly advantageous for managing captive and centralized repositories with non-changing static data. For the enterprise, this approach is best employed to manage captive and centralized data archives and business records repositories, which is what "enterprise search" tools were originally designed to do.
Indexes are used in e-discovery as well both at the point of data collection (desktops, laptops, file and email servers, etc.) and at the back end during the processing, analysis and review phase of the Electronic Discovery Reference Model (EDRM) process. Indexing during processing and review is a highly effective way to optimize a search, evaluate a collection and facilitate review.