The Collaborative International Dictionary of English v.0.48:

Compression \Com*pres"sion\, 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].

   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.
         [PJC]

WordNet (r) 3.0 (2006):

compression
    n 1: an increase in the density of something [syn: compaction,
         compression, concretion, densification]
    2: the process or result of becoming smaller or pressed
       together; "the contraction of a gas on cooling" [syn:
       compression, condensation, contraction]
    3: encoding information while reducing the bandwidth or bits
       required [ant: decompression]
    4: applying pressure [syn: compression, compressing] [ant:
       decompressing, decompression]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "compression":
   abbreviation, abridgment, abstract, agglutination, apocope,
   astriction, astringency, bottleneck, cervix, circumscription,
   clamping, clamping down, clumping, clustering, coarctation,
   compactedness, compaction, compressure, concentration, concretion,
   condensation, consolidation, conspectus, constriction,
   constringency, contraction, contracture, crush, curtailment,
   decrease, densification, diminuendo, elision, ellipsis, epitome,
   foreshortening, hardening, hourglass, hourglass figure, isthmus,
   knitting, narrow place, narrowing, neck, nip, pinch, precis, press,
   pressure, puckering, pursing, recap, recapitulation, reduction,
   retrenchment, shortening, solidification, squeeze, squeezing,
   stranglement, strangulation, striction, stricture, summary,
   summation, syncope, synopsis, systole, telescoping, tightening,
   truncation, tweak, wasp waist, wrinkling



The Free On-line Dictionary of Computing (30 December 2018):

compression
compaction
uncompression

   1.  (Or "compaction") The coding of data to save
   storage space or transmission time.  Although data is already
   coded in digital form for computer processing, it can often be
   coded more efficiently (using fewer bits).  For example,
   run-length encoding replaces strings of repeated characters
   (or other units of data) with a single character and a count.
   There are many compression algorithms and utilities.
   Compressed data must be decompressed before it can be used.

   The standard Unix compression utilty is called compress
   though GNU's superior gzip has largely replaced it.  Other
   compression utilties include pack, zip and PKZIP.

   When compressing several similar files, it is usually better
   to join the files together into an archive of some kind
   (using tar for example) and then compress them, rather than
   to join together individually compressed files.  This is
   because some common compression algorithms build up tables
   based on the data from their current input which they have
   already compressed.  They then use this table to compress
   subsequent data more efficiently.

   See also TIFF, JPEG, MPEG, Lempel-Ziv Welch,
   "lossy", "lossless".

   Compression FAQ
   (ftp://rtfm.mit.edu/pub/usenet/news.answers/compression-faq/).

   Web Content Compression FAQ

(http://perl.apache.org/docs/tutorials/client/compression/compression.html).

   Usenet newsgroups: news:comp.compression,
   news:comp.compression.research.

   2.  Reducing the dynamic range of an audio signal,
   making quiet sounds louder and loud sounds quieter.  Thus,
   when discussing digital audio, the preferred term for reducing
   the total amount of data is "compaction".  Some advocate this
   term in all contexts.

   (2004-04-26)