Source : Webster's Revised Unabridged Dictionary (1913)

Compression \Com*pres"sion\, n. [L. compressio: cf. F.
   compression.]
   The act of compressing, or state of being compressed.
   ``Compression of thought.'' --Johnson.

Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

compression
     
        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 {algorithm}s 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/)}.
     
        {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.
     
        (2001-01-30)