%compression%

. 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 ]