compression, external stress applied to an object or substance, tending to cause a decrease in volume (see pressure). Gases can be compressed easily, solids and liquids to a very small degree if at all. Water, for example, is practically incompressible, thus making it especially useful for hydraulic machines. According to the kinetic-molecular theory of gases, when the molecules of a gas are brought close enough together by compression, the gas (under certain conditions of temperature) undergoes liquefaction. This principle is applied commercially to several gases, including liquid oxygen and the so-called bottled gas (a mixture of hydrocarbons) used as a fuel. Boyle's law deals with the decrease in the volume of a gas in relation to the increase of pressure upon it (see gas laws). The ability or the degree to which an internal-combustion engine reduces the volume of its fuel mixture preparatory to firing is called its compression. Also, a region of high pressure in a fluid is called a compression; thus sound waves are said to propagate at compressions and rarefactions (regions of low pressure) of their medium, such as air.

Process of reducing the amount of data needed for storage or transmission of a given piece of information (text, graphics, video, sound, etc.), typically by use of encoding techniques. Data compression is characterized as either lossy or lossless depending on whether some data is discarded or not, respectively. Lossless compression scans the data for repetitive sequences or regions and replaces them with a single “token.” For example, every occurrence of the word the or region with the colour red might be converted to $. ZIP and GIF are the most common lossless formats for text and graphics, respectively. Lossy compression is frequently used for photographs, video, and sound files where the loss of some detail is generally unnoticeable. JPEG and MPEG (see MP3) are the most common lossy formats.

Learn more about data compression with a free trial on Britannica.com.

Degree to which the fuel mixture in an internal-combustion engine is compressed before ignition. It is defined as the volume of the combustion chamber with the piston farthest out divided by the volume with the piston in the full-compression position (see piston and cylinder). A compression ratio of six means that the action of the piston compresses the mixture to one-sixth its original volume. A high ratio promotes efficiency but may cause engine knock.

Learn more about compression ratio with a free trial on Britannica.com.

Compression may refer to:

In physical science:

• Physical compression, the result of the subjection of a material to compressive stress (the opposite of tension)
• Compression member, a class of structural elements, of which a column is the most common specific example
• Compressibility, the reciprocal of the bulk modulus (typically for solids)
• Gas compression, raising the pressure and reducing the volume of gases
• Compression ratio, a number that predicts the performance of any internal-combustion engine
• Compressibility, a measure of volume change resulting from pressure (typically for fluids)
• Compression (geology), a system of forces that tend to decrease the volume of or shorten rocks

In information science:

• Data compression, the process of encoding information using fewer bits
• Image compression
• Audio data compression
• Video compression
• Compression artifact, noticeable defects in audio or video that has been compressed
• Bandwidth compression, in telecommunications
• Gain compression, in electronic amplifiers, a result of device non-linearity
• Audio level compression, usually artificially introduced gain compression to improve the perceived quality of audio

In other sciences and technologies:

• Compression, in zoology and paleontology, refers to when an animal, or part of an animal, is shorter or narrower compared with other animals in the same group; e.g. the body of a lizard may be compressed (flattened) so it can better fit into crevices under rocks
• Compression (functional analysis), in mathematics
• Compression bandage, designed to reduce the flow of blood

See also

• Compressor (disambiguation)
• compress, a UNIX program
• Compaction
• Decompression
• Expansion
• Compression shorts