It is a disorder. Major affective disorder, depression = s clinical depression which is a treatable mental illness.
Depression. Depression is due to chemical (neurotransmitter) imbalance, which can be triggered by various emotional, social, and phyisiological stress. As your body and mind are so closely connected, you could say depression is a mental condition in itself but at the same time it is a physical condition. Can it cause a mental condition? The answer is yes. When it's severe, it can cause psychosis.
2 general types. Generally there are two types of depression. The first is dysthymia, which is a chronic type of depression that varies little in intensity, and major depression, which varies in intensity a great deal. Essentially depression, in and of itself, IS a "mental condition." That being said it can aggravate and/or be parts of other mental conditions, more correctly termed mental disorders.