What is the term used to describe a belief that is firmly held despite evidence to the contrary, as illustrated by a client believing the FBI is watching him?

The belief that is firmly held despite clear evidence contradicting it is best described as a delusion. In this context, a client believing that the FBI is watching him illustrates a delusion, which is characterized by strongly held false beliefs that are not aligned with reality. Delusions are often encountered in various psychological conditions and can significantly impact an individual’s perception of reality and ability to function.

In contrast, psychosis refers to a broader category of mental health issues that can involve delusions, hallucinations, or a disconnection from reality, but it encompasses more than just delusional beliefs. Hallucinations involve perceiving things that are not present, such as hearing voices or seeing things that do not exist. Neurosis relates to a range of anxiety-related disorders and emotional disturbances, which generally do not include such firmly held false beliefs like the one described.

Therefore, the term "delusion" most accurately captures the nature of the belief in this scenario, highlighting its defined characteristics rooted in psychological terminology.