An attribution bias is a cognitive (thought) bias that refers to the systematic errors that a person makes when they try to find reasons for their own behaviors and motivations as well as the behaviors and motivations of others.
It's normal for people to make attributions (reasons or excuses) for behavior, their own or others.
However these types of attributions are not necessarily an accurate mirror of reality. This type of bias exists as a means of self-protection for the ego because humans have immense difficulty in objectively perceiving their own behaviors. Examples of attribution biases include the Fundamental Attribution Error and the self-serving bias.