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

reference counting

    A garbage collection technique where each
   memory cell contains a count of the number of other cells
   which point to it.  If this count reaches zero the cell is
   freed and its pointers to other cells are followed to
   decrement their counts, and so on recursively.

   This technique cannot cope with circular data structures.
   Cells in such structures refer (indirectly) to themselves and
   so will never have a zero reference count.  This means they
   would never be reclaimed, even when there are no references
   from outside the structure.

   (1995-02-22)