Mutation
A mutation is biological change to monster or more commonly to PC. It might be purely cosmetical (like having changed eye color) or heavily affecting gameplay. Mutations not necessarily have to be bad, sometimes they are neutral for certain player classes, races or even quite beneficial.
There are many ways to alter one's DNA and it is specific for game. Chaos presence, magical contamination, radiation, spells, items infused with distortional energy are just few to be named. Depending on the circumstances strenght and type of change made to subject entity may vary. Eating corpse of high chaos servant probably will twist the PC a lot more than indirect exposition to radioactive area may. Finally, some races have tendency to mutate on advancement.
Mutation may do none, some or all of the following: