Clinical depression, is very specific criteria for what we call a major depression. That basically it means, that the person's level of functioning is affected, whether it be functioning at home or at work due to symptoms that include feeling depressed, feeling anxious, often the person has difficulty concentrating.
They may have sleep disturbance often with insomnia, difficulty sleeping at night, sometimes too much sleep, having a difficult time getting out of bed. Often there is appetite disturbance and weight change sometimes eating too much, often weight loss due to a decrease in appetite, and another very very important symptom is what we call Anhedonia.
It comes from the Greek word Hedonism, and anhedonia is an inability to enjoy oneself. So if a person feels or notices that an activity that they normally would enjoy, such as having catch with the kids or going o the movies with a friend, if they can't enjoy that activity, there's a good chance that they have depression.