ISAM

Indexed Sequential Access Method (ISAM) is a method for indexing data for fast retrieval that was originally developed by IBM for mainframe computers. ISAM allows database records to be accessed either sequentially as they were created or randomly via an index which in turn represents the same data in a different order. This is in contrast to a relational database that uses query optimising software that automatically creates and chooses indices.

Most modern databases systems that offer ISAM use a B-Tree as the back end for their system, IBM's ISAM and later VSAM (Virtual Storage Access Method) implementations do not and programming languages that offer ISAM implementations usually offer a simpler but more flexible systems.

Links
Robert L. Bogue: Explore the differences between ISAM and relational databases - from TechRepublic