Lossy compression
From The Glossary of Digital Photography
A data compression technique in which duplicate or near duplicate data are tossed out to reduce the size of a file. In the case of digital images, the very popular JPEG image file format is a lossy compression technique. The image, when uncompressed from a lossy compression technique such as JPEG, is visually similar to the original image, but not exactly the same. For many uses, such as web images, the loss of quality is worth the large storage-space savings and the reduced download times. The amount of compression—and therefore of quality—is user selectable in most cases. The more compression chosen, the lower the quality of the compressed image. When using lossy compression file formats, it is important that the image not be repeatedly resaved. Each resaving operation will reduce the quality. See also lossless compression; JPEG
