Complex sentence: Bipolar II disorder is a bipolar spectrum disorder characterized by at least one hypomanic episode and at least one major depressive episode; with this disorder, depressive episodes can be more frequent and are more intense than hypomanic episodes.
Split into simpler sentences: Bipolar II disorder is a bipolar spectrum disorder characterized by at least one hypomanic episode and at least one major depressive episode. The defining feature, hypomania with this disorder, depressive episodes can be more frequent and are more intense than hypomanic episodes.