Minor depression, which some people might call everyday depression, is a diagnosable mental illness. It is not a condition that will always disappear without treatment. It is a medical condition that people need to get treatment for, so they have the best chance of achieving a full recovery.
In an overview article in the Primary Care Companion, Journal of Clinical Psychiatry, researchers say that a diagnosis of minor depression applies when people have at least two symptoms of depression, but fewer than five symptoms. Those symptoms must persist during a two-week period.
People like this are definitely struggling with their mental health. But they may not get help for those issues, because they simply do not seem severe enough to merit attention. People like this may still be able to go to work, handle home obligations, and otherwise lead a relatively normal life, but deep down, they are struggling and they need help.