‘Depression’ is often used to refer to major depressive disorder, sometimes also called clinical depression. Depression is different than feeling sad for a short period of time.
Clinical depression is a common and serious medical condition, that can be treated effectively. It is generally characterized by an individual feeling down most of the time and/or losing pleasure in regular activities, for a period of two or more weeks.
Major depressive disorder is the most common mood disorder, but it is only one of a number of others.