ISAM stands for Indexed Sequential Access Method, a method for indexing data for fast retrieval. ISAM was originally developed by IBM for mainframe computers. Today the term is used for several related concepts:

• Specifically, the IBM ISAM product and the algorithm it employs.
• A database system where an application developer directly uses an application programming interface to search indexes in order to locate records in data files. In contrast, a relational database uses a query optimizer which automatically selects indexes.
• An indexing algorithm that allows both sequential and keyed access to data. Most databases use some variation of the B-Tree for this purpose, although the original IBM ISAM and VSAM implementations did not do so.
• Most generally, any index for a database. Indexes are used by almost all databases, both relational and otherwise.