People with depression experience symptoms that affect their mood, cognitive function and physical health. from www.shutterstock.com, CC BY-ND Depression: it’s a word we use a lot, but what exactly is it? Samuel Clack, Victoria University of Wellington and Tony Ward, Victoria University of Wellington Depression is a serious disorder marked by disturbances in mood, cognition, physiology and social functioning. People can experience deep sadness and feelings of hopelessness, sorrow, emptiness and despair. These core features of depression have expanded to include an inability to experience pleasure, sluggish movements, changes in sleep and eating behaviour, difficulty concentrating and suicidal thoughts. The first diagnostic criteria were introduced in the 1980s. Now we have an expanded set of concepts for describing depression, from mild to severe, major depressive disorder, chronic depression and seasonal affective disorder. Over the past 50 years, our understanding of depression has advanced significantly. But despite the wealth of research, there is no clear consensus on how this mental disorder should be explained. We propose a new route through the thicket. Read more: What causes depression? What we know, don’t know and suspect Classifying mental disorders How we describe and classify mental disorders is a fundamental step towards explaining and…