In computer graphics, pixelation is caused by displaying a bitmap or a section of a bitmap at such a large size that individual pixels, small single-colored square display elements that comprise the bitmap, are visible.
A compression artifact is a noticeable distortion of media caused by the application of lossy data compression. Lossy data compression involves discarding some of the media's data so that it becomes simplified enough to be stored within the desired disk space. If the compressor could not reproduce enough data in the compressed version to reproduce the original, the result is a diminishing of quality, or introduction of artifacts. Alternatively, the compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the viewer.