Define the term mental functional disorders?

Functional disorders: "functional" disorders is a vague term used to describe certain persistent symptoms for which no other organic or pathologic cause can be found. One example, would be in some one who has chronic headaches or abdominal pain, and has had a full set of testing and evaluation procedures, with no diagnosis. Other more unusual symptoms include: paralysis, pseudo-seizures, certain tics, vomiting, etc.