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

Physical Sequential
QSAM
Queued Sequential Access Method

    (PS, QSAM, Queued Sequential Access Method) The
   simplest data set on an IBM mainframe.  Sequential files
   can only be read or written from the beginning: they do not
   support random access.

   [Why "Queued"?]

   (2003-12-05)