run-length encoding 
A compression technique that reduces the size of bitmapped graphics files, especially for black-and-white or cartoon-style line art graphics. It works by replacing "runs" of the same color with a single character. The more runs there are, and the longer the run sequence, the greater the compression. A file compressed this way often has the file extension .rle, but the technique can be used on PCX, TIFF, and BMP files as well.