n. [ L. compressio: cf. F. compression. ] 1. The act of compressing, or state of being compressed. “Compression of thought.” Johnson. [ 1913 Webster ]

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 { 3 }. 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. [ PJC ]

. A projectile constructed so as to take the grooves of a rifle by means of a soft copper band firmly attached near its base or, formerly, by means of an envelope of soft metal. In small arms the modern projectile, having a soft core and harder jacket, is subjected to compression throughout the entire cylindrical part. [ Webster 1913 Suppl. ]

n. 1. the process of experiencing decompression; the act or process of relieving or reducing pressure.
Syn. -- decompressing. [ WordNet 1.5 +PJC ]

2. the reduction of atmospheric pressure experienced by divers rising from deep water to the surface, thus reducing the concentration of dissolved atmospheric gases in the blood; -- especially applied to a gradual reduction of such pressure. [ PJC ]

3. the process, analogous to sense 2, undergone by divers in a decompression chamber, in which an artificially high atmospheric pressure is gradually lowered to normal pressure. [ PJC ]

4. a return to a normal, more relaxed state after a period of intense stress, psychological pressure, or urgent activity; -- of people. [ PJC ]

5. (Computers) the process of converting digitally encoded data from a more compact (compressed) form to its original, larger size. The process of compression and decompression may completely recover all of the original data (called lossless compression), or may lose some of the original data in order to achieve higher degress of compression (lossy compression). The latter is used especially with images or video data, which may be of very large size relative to text, and for which small changes may be imperceptible to the human eye. The JPEG data compression format is a lossy format. [ PJC ]

n. (Computers) The compression of binary data into a form which, when it is re-expanded, has most, but not all, of the original information. It is used primarily for compression of images and sounds, and is designed to provide a high degree of compression at the cost of a slight loss of data. It is expemplified by the JPEG compression standard. Images compressed by a lossy compression algorithm are re-expanded into an image close, but not identical to the original image; the difference between the original and the reconstructed image may be imperceptible to normal viewing by the eye. [ PJC ]