These creatures did not fit into the natural world.
Aberrations generally all had bizarre anatomies, strange abilities, alien mindsets, or any combination thereof.
They are usually strange and alien in nature, with unusual anatomies, unusual powers, and unnatural behaviors and motivations. Aberrations can have any number of origins, from being the result of magical experiments, to being the spawn of powerful extra-dimensional creatures, to being the result of natural evolution.
The overarching characteristic that all aberrations shared was that they did not fit into the natural order of things. They had no natural predators, and no natural creature had the instincts to avoid them. Worse, aberrant powers warped and corrupted the area where they dwelled.