An **entropy encoding** is a coding scheme that assigns codes to symbols so as to match code lengths with the probabilities of the symbols.
Typically, entropy encoders are used to compress data by replacing symbols represented by equal-length codes with symbols represented by codes proportional to the negative logarithm of the probability.
Therefore, the most common symbols use the shortest codes.

According to Shannon's theorem, the optimal code length for a symbol is -log* _{b}P*, where

Three of the most common entropy encoding techniques are Huffman coding, Range encoder and arithmetic encoding. If the approximate entropy characteristics of a data stream are known in advance (especially for signal compression), a simpler static code such as Unary coding, Elias Gamma coding, Fibonacci coding, Golomb coding, or Rice coding may be useful.

An earlier version of the above article was posted on PlanetMath. This article is open content.

*See also:* entropy, Universal code