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

Keyed Sequenced Data Set

    (KSDS) One of the access methods used by VSAM.
   KSDS has indexes and data split into CI (Control Interval) in
   CA (Control Area) and multi index levelled.  Forward and
   backward compression is applied to key values.

   (1999-01-11)