Mental disorder is not easy to define. It is a broad area and there is no single definition in the literature that covers all the situations and medical conditions that characterize a mental disorder. A person can be categorized a having a disorder (physical or mental) on the basis of structural pathology, symptom presentation, deviance from a physiological norm, and etiology.
Different researchers have attempted to define a mental disorder on the basis of concepts such as distress, disability, irritability, inflexibility, irrationality etc. Each of these can indicated a mental disorder, however, a comprehensive definition is still lacking. DSM has attempted to define a mental disorder as:
“ a clinically significant behavioral or psychological syndrome or pattern that occurs in an individual and that is associated with present distress (e.g., a painful symptom) or disability (i.e., impairment in one or more important areas of functioning) or with a significantly increased risk of suffering death, pain, disability, or an important loss of freedom. In addition, this syndrome or pattern must not be merely an expectable and culturally sanctioned response to a particular event, for example, the death of a loved one. Whatever its original cause, it must currently be considered a manifestation of a behavioral, psychological, or biological dysfunction in the individual. Neither deviant behavior (e.g., political, religious, or sexual) nor conflicts that are primarily between the individual and society are mental disorders unless the deviance or conflict is a symptom of a dysfunction in the individual”.
So I think any condition that hinders a person from doing their day-to-day duties can be classified as mental disorder, such as depression, where a person loses all his motivation to live fully.