Lossy compression, or irreversible compression, is a data-compression method that uses inexact approximations and partial-data discarding to represent content.
Lossy compression is used in media formats such as JPEG. When images are not compressed, three numbers, one for each of the red, green and blue components, are needed for each pixel, leading to huge file sizes. Lossy compression means that some data from the original image file is lost. This process is irreversible. Images that were compressed this way should generally not be further edited.