V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ISAM
       Index-Sequential Access Mode (DB, SAM)


The Free On-line Dictionary of Computing (30 December 2018):

Indexed Sequential Access Method
ISAM

    (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)