Depression (major depressive disorder) is a common and serious medical illness that negatively affects how you feel, the way you think and how you act. Fortunately, it is also treatable. Depression causes feelings of sadness and/or a loss of interest in activities you once enjoyed. It can lead to a variety of emotional and physical problems and can decrease your ability to function at work and at home.

• Feeling sad or having a depressed mood
• Loss of interest or pleasure in activities once enjoyed
• Changes in appetite — weight loss or gain unrelated to dieting
• Trouble sleeping or sleeping too much
• Loss of energy or increased fatigue
• Increase in purposeless physical activity (e.g., inability to sit still, pacing, handwringing) or slowed movements or speech (these actions must be severe enough to be observable by others)
• Feeling worthless or guilty
• Difficulty thinking, concentrating or making decisions
• Thoughts of death or suicide

Depression symptoms can vary from mild to severe and can include:

Symptoms must last at least two weeks and must represent a change in your previous level of functioning for a diagnosis of depression.

Reference

https://www.psychiatry.org/patients-families/depression/what-is-depression