Stochastic Process:   

Any process whereby selection of individual components are random, yet the global characteristic predictable within certain parameters. Generally, stochastic processes are applied in music to allow local elements to be indeterminate while keeping a level of control over the global parameters. 
In Soundhack's mutation functions, the irregular mutations utilize stochastic processes to determine which frequency bands will be processed.  For more information on the application of stochastic processes in music see either Xenakis's Formalized Music,
or James Tenney Meta Hodos,
or articles by Larry Polansky.