The term mental disorder is heavily used in psychiatric medicine, and is defined in some psychiatric medicine texts, most notably the Diagnostic and Statistical Manual of Mental Disorders (DSM).
The term "disorder" bears no special clinical relevance, and could be used interchangeably with disease.
The use of the term "disorder" likely rests on historical precedent as well as the preference of the field.
Similarly, disorders are typically not acquired, or the result of environmental factors, such as lung disease.
A disease or illness described as a physical disorder likely impacts the musculoskeletal system and lacks an inciting injury.