A mixed episode is not a disorder itself, but rather is a description of a type of bipolar disorder.
A mixed epsiode is defined by meeting the diagnostic criteria for both a manic episode as well as a major depressive episode nearly every day for at least a full week.
Like most mental disorders, a mixed episode must be severe enough to cause distress or impairment in social, occupational, education or other important functioning and is not better accounted for by the physiological effects of substance use or abuse (alcohol, drugs, medications) or a general medical condition.