Is depression a psychiatric disorder?

No, it's a mood. It's a bit of a trick question. In dictionaries you can find a definition like "the state of being depressed." everyone feels depressed now and then--it's a human emotion. But, when a depressed mood lasts for 2 weeks or longer and is accompanied by a constellation of other symptoms a person can be thought of as having a depressive "disorder." this is a treatable condition.
Major Depression is- "depression, " is a symptom, like fever is a symptom. There are specific diagnostic criteria for: major depression as well as other, related mood disorders, which are considered psychiatric conditions. But, there can also be physical causes to explain mood disturbances, for example- low thyroid levels. You should probably start by seeing your local medical doctor, and go from there.