Term first used by Horst Feistel
The avalanche effect refers to a desirable property of cryptographic algorithms, typically block-ciphers and hash functions. The avalanche effect is evident if, when an input is changed slightly (for example, flipping a single bit) the output changes significantly.
Constructing a cipher to exhibit a substantial avalanche effect is one of the primary design goals. If a block cipher does not exhibit the avalanche effect to a significant degree, then it has poor randomization, and thus a cryptanalyst can make predictions about the input, being given only the output. This may be sufficient to partially or completely break the algorithm.