Strict Avalanche Criterion and Avalanche criterion?
The definition of avalanche effect is given in the paper of Webster, A. F. "On the design of S-boxes". Advances in Cryptology - Crypto '85 as :


  For a given transformation to exhibit the avalanche effect, an average of one half of the output bits should change whenever a single input bit is complemented.


It is also seen as each bit should have 50% chances to change if you change 1 bit of the input. (strict avalanche)



In the avalanche criterion you look at the output as a whole (average 50% of the bits changes).

In the strict avalanche criterion, you look at each bit one by one and you verify that what ever the other bits will change, it will have a 50% probability to switch.

Related questions:


Hash functions and the Avalanche effect
Calculation of the avalanche effect coefficient