Lossy

Lossy is a term describing a data compression algorithm which actually reduces the amount of information in the data, rather than just the number of bits used to represent that information.

Description
The lost information is typically removed by a lossy algorithm because it is subjectively less important to the quality of the data (usually an image or sound) or because it can be recovered reasonably by interpolation from the remaining data.

MPEG and JPEG are examples of lossy compression techniques. An algorithm which mathematically loses no such information is described as lossless.