In  cryptography,  the  avalanche  effect  refers  to  a
desirable  property  of  cryptographic  algorithms,
typically  block  ciphers  and  cryptographic  hash
functions. The avalanche effect is evident if, when an
input is changed slightly (for example, flipping a single
bit)  the  output  changes  significantly  (e.g.,  half  the
output  bits  flip).  In  the  case  of  high-quality  block
ciphers, such a small change  in either  the key  or the
plaintext  should  cause  a  drastic  change  in  the
ciphertext.
 
If a  block cipher or  cryptographic hash function 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. Thus,  the  avalanche
effect is a desirable condition from the point of view of
the designer of the cryptographic algorithm or device.

(PDF) Study of Avalanche Effect in AES. Available from: https://www.researchgate.net/publication/304141002_Study_of_Avalanche_Effect_in_AES [accessed Aug 23 2018].