What is the meaning of compression?

noun

• An increase in the density of something.
  • synonyms: compaction, concretion, densification
• The process or result of becoming smaller or pressed together.
  • usage: "the contraction of a gas on cooling"
  • synonyms: condensation, contraction
• Encoding information while reducing the bandwidth or bits required.
• Applying pressure.
  • synonyms: compressing