According to the American Psychological Association (APA), “depression is extreme sadness or despair that lasts more than days. It interferes with the activities of daily life and can cause physical symptoms such as pain, weight loss or gain, sleeping pattern disruptions, or lack of energy. People with depression may also experience an inability to concentrate, feelings of worthlessness or excessive guilt, and recurrent thoughts of death or suicide. Fortunately, depression is treatable. A combination of therapy and antidepressant medication can help ensure recovery.” 

HelpGuide identified a list of symptoms and warning signs of depression. They are as follows:

1. Feelings of helplessness and hopelessness. A bleak outlook—nothing will ever get better and there's nothing you can do to improve your situation. 
2. Loss of interest in daily activities. You don't care anymore about former hobbies, pastimes, social activities, or sex. You've lost your ability to feel joy and pleasure. 
3. Appetite or weight changes. Significant weight loss or weight gain—a change of more than 5% of body weight in a month. 
4. Sleep changes. Either insomnia, especially waking in the early hours of the morning, or oversleeping. 
5. Anger or irritability. Feeling agitated, restless, or even violent. Your tolerance level is low, your temper short, and everything and everyone gets on your nerves. 
6. Loss of energy. Feeling fatigued, sluggish, and physically drained. Your whole body may feel heavy, and even small tasks are exhausting or take longer to complete. 
7. Self-loathing. Strong feelings of worthlessness or guilt. You harshly criticize yourself for perceived faults and mistakes. 
8. Reckless behavior. You engage in escapist behavior such as substance abuse, compulsive gambling, reckless driving, or dangerous sports. 
9. Concentration problems. Trouble focusing, making decisions, or remembering things. 10. Unexplained aches and pains. An increase in physical complaints such as headaches, back pain, aching muscles, and stomach pain. 

According to the Mayo Clinic, people with depression may not recognize or acknowledge that they're depressed. They may not be aware of signs and symptoms of depression, so they may think their feelings are normal. Here's what you can do to help: 

• Talk to the person about what you've noticed and why you're concerned. 
• Explain that depression is a medical condition, not a personal flaw or weakness — and that it usually gets better with treatment. 
• Suggest seeking help from a professional — a medical doctor or a mental health provider, such as a licensed counselor or psychologist. 
• Offer to help prepare a list of questions to discuss in an initial appointment with a doctor or mental health provider. 
• Express your willingness to help by setting up appointments, going along with them and attending family therapy sessions. 

References:

https://www.apa.org/topics/depression 

https://dictionary.apa.org/depression 

https://www.mayoclinic.org/diseases-conditions/depression/in-depth/depression/art-20045943 

https://www.helpguide.org/articles/depression/depression-symptoms-and-warning-signs.htm