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

average seek time

    The mean time it takes to move the head of a
   disk drive from one track to another, averaged over the
   source and destination cylinders.  Usually measured in
   milliseconds (ms).

   The average seek time gives a good measure of the speed of the
   drive in a multi-user environment where successive read/write
   request are largely uncorrelated.

   Ten ms is common for a hard disk and 200 ms for an
   eight-speed CD-ROM.

   (2007-03-16)