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

Virtual Storage Access Method
Virtual Sequential Access Method
VSAM

    (VSAM) An IBM disk file storage scheme first used
   in S/370 and virtual storage.  VSAM comprises three access
   methods: Keyed Sequenced Data Set (KSDS), Relative Record
   Data Set (RRDS), and Entry Sequenced Data Set (ESDS).

   Both IMS/DB and DB2 are implemented on top of VSAM and use
   its underlying data structures.

   ["Storage" or "Sequential"?]

   (2002-07-26)