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  1. Reducing the dynamic range of a signal by electrical circuits that reduce the level of loud passages.
  2. In audio, compression means to reduce the dynamic range of a signal. Compression may be intentional or one of the effects of a system that is driven to overload. It is also the portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure. Source: http://www.owenscorning.com/around/sound/glossary.asp