The word is often used lightly. There are many different levels of depression. Clinical depression is serious, and could ultimately be life threatening. Clinical depression is defined as a mood disorder that manifests intense sadness, beyond what is normally expected.
Symptoms can be both physical and emotional.
The person is unable to function on a daily basis. They usually have insomnia or sleep all the time, anorexia, possible weight loss or they may overeat and gain weight, irritability, fears and anxieties, apathy, low self-esteem and trouble concentrating and carring out activities of daily living, such as working, cooking etc. They usually don't want to be around any family or friends. They may have suicidal thoughts. Depression doesn't play favorites.