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

run-length encoding
RLE

   A kind of compression algorithm which replaces sequences
   ("runs") of consecutive repeated characters (or other units of
   data) with a single character and the length of the run.  This
   can either be applied to all input characters, including runs
   of length one, or a special character can be used to introduce
   a run-length encoded group.  The longer and more frequent the
   runs are, the greater the compression that will be achieved.
   This technique is particularly useful for encoding black and
   white images where the data units would be single bit
   pixels.

   (1994-10-27)