"Clinical depression" refers to Major Depressive Disorder, which is the name given to the common set of psychiatric dysfunctions that have persistent depression

"Clinical depression" refers to Major Depressive Disorder, which is the name given to the common set of psychiatric dysfunctions that have persistent depression as their primary symptom. That is, "Clinical depression" refers to an entire disorder (a set of symptoms grouped together in the literature) rather than as an individual symptom. This is to distingish it from "depression" used as the name of a symptom or other disorders, like bipolar. "Clinical" depression is not depression that is caused by chemistry rather than psychology. We have no idea whose depression is caused by what, really, and chemistry/psychology probably isn't a meaningful distinction to make in this context.