How does psychology define abnormal behavior?
Psychology, like any other field of study, establishes what abnormal by first establishing what is normal. This answer might seem like an easy answer, but the implications of this are very important. First, this definition underlines that what is considered abnormal differs from time to time and from place to place. For example, a person who is obsessed with dream may be considered out of touch with reality today, but if you lived during the ancient or medieval world, this would not be classified as abnormal behavior, because everyone knew that one of the ways that God spoke was through dreams! Also Foucault has shown that "mentally ill" patients were only considered so when the medical establishment came into power. Before that some of the "mentally ill" were considered liminal figures of great power. Finally, I am not suggesting that there is nothing that could be classified as abnormal, but I am stating that we need to hesitate a bit, because knowledge and classifications are socially constructed.
In the field of psychology each culture has its own established set of social, intellectual, adaptive, and emotional norms. For someone's behavior to be abnormal it needs to be different that a the mean of the population. It must deviate from a population's statistical norms. Most people think of abnormal behavior as having only negative connotations but a person who highly intelligent is considered to be abnormal. However, the majority of people identify abnormal behavior as maladaptive behavior.