A form of image compression that reduces repeating elements in an image to mathematical expressions. Unlike JPEG compression, LZW compression is "lossless;" that is, compressing an image with LZW does not cause any image degradation. TIFF images can be LZW compressed; GIF images are always LZW compressed. (LZW is named for Lempel-Ziv-Welsh, the mathematicians who jointly invented it.)
Lempel, Zif, Welch compression. A form of run-length encoding that compresses some bitmap images. Compression is carried out by an algorithm that looks for areas of a single colour or patterns and replaces the repeating pixel data with the equivalent of 'the next pixels are...' Of little use for compressing continuous tone bitmaps and may in fact enlarge them.
Lempel-Ziv-Welch is a proprietary lossless data-compression algorithm used in GIF and TIFF files.