ISAM

from The Free On-line Dictionary of Computing (8 July 2008)

Indexed Sequential Access Method
ISAM

   <database> (ISAM) An {IBM} file management system allowing
   records to be accessed either sequentially (in the order they
   were entered) or via an index.  Each index orders the records
   on a different key.

   ISAM was followed by VSAM ({Virtual Storage Access Method})
   and pre-dated {relational databases}.

   (2003-07-13)