Major depressive disorder with psychotic features, also called psychotic depression, is one of the most severe forms of depression. This is diagnosed when a person has all the symptoms of depression, is too distressed to function properly and has the added symptoms of psychosis.

Psychosis is generally defined as a break from reality. For example, a person living with psychosis may believe things that are not true despite being shown facts that argue against those beliefs, without those beliefs being part of their culture.

Major depressive disorder without psychotic features is an illness that causes a depressed mood or a lack of interest and enjoyment in things, along with a number of other symptoms such as low energy, changes in appetite, sleep problems, thoughts of death or negative thoughts about oneself or the world. It lasts for many months.

Once called dysthymic disorder, persistent depressive disorder does not meet the full criteria for a major depressive episode. However, it persists for a long time: at least two years of symptoms without much relief during that time. Typically, the person can function, but they don’t feel well, they don’t feel happy, they don’t enjoy things and they have some other physical or cognitive symptoms.

Subsyndromal, subthreshold or minor depression are forms of depression that do not meet the criteria for the other types of depression, but they still come with an impaired quality of life and greater healthcare utilization. The conversion rate from these forms of depression to major depression is about 10 per cent per year.

In adjustment disorders, people have experienced some sort of stress that causes them to feel sad. This can be normal at times—for example, feeling sad after someone dies. However, it is important to be mindful that people who are experiencing stressors, have an adjustment problem or adjustment disorder, or are experiencing bereavement may need intervention at some point if they develop one of the other depressive disorders.