Run length encoding (RLE)
From The Glossary of Digital Photography
A non-lossy compression algorithm. RLE is used in BMP and GIF files, among others. It is also used to compress animation stills. See also non-lossy; GIF; BMP

ONLINE GLOSSARY OF DIGITAL PHOTOGRAPY
A non-lossy compression algorithm. RLE is used in BMP and GIF files, among others. It is also used to compress animation stills. See also non-lossy; GIF; BMP