Paranoia is a symptom in which an individual feels as if the world is "out to get" him or her. When people are paranoid, they feel as if others are always talking about them behind their backs. Paranoia causes intense feelings of distrust, and can sometimes lead to overt or covert hostility.
An individual suffering from paranoia feels suspicious, and has a sense that other people want to do him or her harm. As a result, the paranoid individual changes his or her actions in response to a world that is perceived as personally threatening. Objective observers may be quite clear on the fact that no one's words or actions are actually threatening the paranoid individual. The hallmark of paranoia is a feeling of intense distrust and suspiciousness that is not in response to input from anybody or anything in the paranoid individual's environment.
You can read more here.
