The Collaborative International Dictionary of English v.0.48:
Compression \Com*pres"sion\, n. [L. compressio: cf. F.
1. The act of compressing, or state of being compressed.
"Compression of thought." --Johnson.
2. (Computers) reduction of the space required for storage
(of binary data) by an algorithm which converts the data
to a smaller number of bits while preserving the
information content. The act of compressing .
Note: Compression may be lossless compression, in which all
of the information in the original data is preserved,
and the original data may be recovered in form
identical to its original form; or lossy compression,
in which some of the information in the original data
is lost, and decompression results in a data form
slightly different from the original. Lossy
compression is used, for example, to compress audio or
video recordings, and sometimes images, where the
slight differences in the original data and the data
recovered after lossy compression may be
imperceptable to the human eye or ear. The JPEG
format is produced by a lossy compression algorithm.