The Collaborative International Dictionary of English v.0.48:

decompression \decompression\ 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.

   Note: 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]

WordNet (r) 3.0 (2006):

decompression
    n 1: restoring compressed information to its normal form for use
         or display [ant: compression]
    2: relieving pressure (especially bringing a compressed person
       gradually back to atmospheric pressure) [syn:
       decompression, decompressing] [ant: compressing,
       compression]