Cliche used in various forms of fiction (usually of the science fiction genre) where a mad scientist type character is trying to force humanity to evolve through unethical means, or into some kind of posthuman monster race.
Doctor Mercer trying to help propagate the Necromorphs was part of the evilution cliche in Dead Space.
A corruption of the word evolution primarily used online by creationisttheists who are ignorant of what evolution actually is. Evilution is most often used in an attempt to portray evolution as negative and anti-religion and can generally be considered a sign that the person using it wouldn't know the difference between a genome and a g-spot.
Creationist: "Evilution has never been proved, it's all a secular lie invented by Darwin to avoid God! You'll burn in Hell for believing in evilution LOLZ!"