Compression artifact

A compression artifact (or artefact) is a deliberate visible change in media (including images, audio, and video). It is caused by lossy compression. Lossy data compression removes some of the media's data. This way the data becomes small enough to be stored in the desired disk or transmitted inside the available bandwidth. If the compressor cannot store enough data in the compressed form, the result is a loss of quality, or introduction of artifacts.
The most common digital compression artifacts are DCT blocks. This is caused by the discrete cosine transform (DCT) compression algorithm used in many digital media, such as JPEG image compression, MP3 digital audio, and MPEG video coding formats. 
These compression artifacts appear when heavy compression is applied. It occurs often in common digital media, such as DVDs, common computer file formats such as JPEG, MP3 and MPEG files. It also occurs in some other alternatives to the compact disc like the as Sony's MiniDisc format. Uncompressed media (such as on Laserdiscs and WAV files) or losslessly compressed media (such as FLAC or PNG) do not suffer from compression artifacts.