Acording to its definition:

"Fuzz testing or fuzzing is a software testing technique, often automated or semi-automated, that involves providing invalid, unexpected, or random data to the inputs of a computer program."

In a simplifed formalization, let $p$ be a program and $i$ an input for that program, after a mutation $m$ is selected, the testing process can be abstracted with a function $f$ ssss $\checkmark$ $\times$ The fuzzing is a process to select:

The function test can be defined as: $$test(p,m,i) = \mbox{ if } p(m(i)) \mbox{ finish } = \checkmark) \mbox{ then ... else } $$