"is a depressed mood that lasts for at least two years. A person diagnosed with persistent depressive disorder may have episodes of major depression along with periods of less severe symptoms, but symptoms must last for two years to be considered persistent depressive disorder." (1)

"occurs when a person has severe depression plus some form of psychosis, such as having disturbing false fixed beliefs (delusions) or hearing or seeing upsetting things that others cannot hear or see (hallucinations). The psychotic symptoms typically have a depressive 'theme,' such as delusions of guilt, poverty, or illness." (1)

"is characterized by the onset of depression during the winter months, when there is less natural sunlight. This depression generally lifts during spring and summer. Winter depression, typically accompanied by social withdrawal, increased sleep, and weight gain, predictably returns every year in seasonal affective disorder." (1)

"is different from depression, but it is included in this list because someone with bipolar disorder experiences episodes of extremely low moods that meet the criteria for major depression (called 'bipolar depression'). But a person with bipolar disorder also experiences extreme high – euphoric or irritable – moods called 'mania' or a less severe form called 'hypomania.'" (1)