A mental disorder or mental illness is a psychological pattern, potentially reflected in behavior, that is generally associated with distress or disability, and which is not considered part of normal development of a person's culture. Mental disorders are generally defined by a combination of how a person feels, acts, thinks or perceives. This may be associated with particular regions or functions of the brain or rest of the nervous system, often in a social context. The recognition and understanding of mental health conditions have changed over time and across cultures, and there are still variations in definition, assessment and classification, although standard guideline criteria are widely used.