First, depression is more than just feeling sad. Depression is a mental disorder that will cause a significant effect on a person's life. For a condition to be considered a mental disorder, it must cause a disturbance in an individual's cognition, emotional regulation, or behavior. Meaning, that when depression is present, it will negatively impact a person's thoughts, feelings, and behaviors.
A key difference between depression vs sadness is that depression will always cause a change in function. This means that when someone is depressed, they will no longer be able to do things that they normally enjoy or are able to do. For example, a person who is depressed may no longer find joy in activities they once loved, may struggle to concentrate or focus on anything, or may start to isolate themselves from friends and family.
Among mental health disorders, research indicates that depression is one of the most common in the United States. The National Institute of Mental Health reports that in 2020, around 21 million adults in the United States, ages 18 and older, had at least one major depressive episode. This is the equivalent of 8.4% of the population.