Which disorder is described as unipolar depression?

Unipolar depression refers to a mood disorder where an individual experiences only depressive episodes without any episodes of mania or hypomania. Major Depressive Disorder (MDD) is central to this definition, characterized by persistent feelings of sadness, hopelessness, and a lack of interest or pleasure in daily activities that lasts for at least two weeks. This condition does not include episodes of elevated mood typical of bipolar disorders, which is why it is categorized distinctly as "unipolar."

The other types of disorders listed involve mood fluctuations that include both depressive and manic or hypomanic episodes, which are crucial for their diagnoses. For instance, Bipolar I Disorder and Bipolar II Disorder both include at least one episode of mania or hypomania, whereas Cyclothymic Disorder is characterized by numerous periods of hypomanic symptoms along with numerous periods of depressive symptoms that do not meet the full criteria for a major depressive episode. Thus, these disorders are not classified as unipolar, distinguishing Major Depressive Disorder as the correct choice in the context of the question.