Straying or differing from the right, normal, or natural type.
Example:
Richard's aberrant behavior began to make his colleagues fear that the stress of the project was getting to be too much for him.
Something that is aberrant has wandered away from the usual or normal path or form. Aberrant behavior is usually bad behavior and may be a symptom of other problems. However, in biology, the discovery of an aberrant variety of a species can be exciting news, and in medical research the discovery of an aberrant gene can lead the way to new cures for diseases.