Depression (also called major depressive disorder, or unipolar depression when compared to bipolar disorder) is a state of intense sadness, melancholia or despair that has advanced to the point of being disruptive to an individual's social functioning and/or activities of daily living.

Although a low mood or state of dejection that does not affect functioning is often colloquially referred to as depression, clinical depression is a clinical diagnosis and may be different from the everyday meaning of "being depressed." Many people identify the feeling of being depressed as "feeling sad for no reason", or "having no motivation to do anything." One suffering from depression may feel tired, sad, irritable, lazy, unmotivated, and apathetic. Clinical depression is generally acknowledged to be more serious than normal depressed feelings. It often leads to constant negative thinking and sometimes substance abuse.

Without careful assessment, delirium can easily be confused with depression and a number of other psychiatric disorders because many of the signs and symptoms are conditions present in depression, as well as other mental illnesses including dementia and psychosis.
Major clinical depression
Major Depression, or, more properly, Major Depressive Disorder (MDD), is characterized by a severely depressed mood that persists for at least two weeks. Major Depressive Disorder is specified as either "a single episode" or "recurrent"; periods of depression may occur as discrete events or recur over the lifespan. Episodes of major or clinical depression may be further divided into mild, major or severe. Where the patient has already had an episode of mania or markedly elevated mood, a diagnosis of bipolar disorder (also called bipolar affective disorder) is usually made instead of MDD; depression without periods of elation or mania is therefore sometimes referred to as unipolar depression because the mood remains on one pole. The diagnosis also usually excludes cases where the symptoms are a normal result of bereavement.

Diagnosticians recognize several possible subtypes of Major Depressive Disorder. ICD-10 does not specify a melancholic subtype, but does distinguish by presence or absence of psychosis.

Depression with Melancholic Features - Melancholia is characterized by a loss of pleasure (anhedonia) in most or all activities, a failure of reactivity to pleasurable stimuli, a quality of depressed mood more pronounced than that of grief or loss, a worsening of symptoms in the morning hours, early morning waking, psychomotor retardation, anorexia (excessive weight loss, not to be confused with Anorexia Nervosa), or excessive guilt.
Depression with Atypical Features - Atypical Depression is characterized by mood reactivity (paradoxical anhedonia) and positivity, significant weight gain or increased appetite, excessive sleep or somnolence (hypersomnia), leaden paralysis, or significant social impairment as a consequence of hypersensitivity to perceived interpersonal rejection. Contrary to its name, atypical depression is the most common form of depression.
Depression with Psychotic Features - Some people with Major Depressive or Manic episode may experience psychotic features. They may be presented with hallucinations or delusions that are either mood-congruent (content coincident with depressive themes) or non-mood-congruent (content not coincident with depressive themes). It is clinically more common to encounter a delusional system as an adjunct to depression than to encounter hallucinations, whether visual or auditory.

Other categories of depression
Dysthymia is a long-term, mild depression that lasts for a minimum of two years. There must be persistent depressed mood continuously for at least two years. By definition the symptoms are not as severe as with Major Depression, although those with Dysthymia are vulnerable to co-occurring episodes of Major Depression. This disorder often begins in adolescence and crosses the lifespan. People who are diagnosed with major depressive episodes and dysthymic disorder are diagnosed with double depression. Dysthymic disorder develops first and then one or more major depressive episodes happen later.

Bipolar I Disorder is an episodic illness in which moods may cycle between mania and depression. In the United States, Bipolar Disorder was previously called Manic Depression. This term is no longer favored by the medical community, however, even though depression plays a much stronger (in terms of disability and potential for suicide) role in the disorder. "Manic Depression" is still often used in the non-medical community. Bipolar II Disorder is an episodic illness that is defined primarily by depression but evidences episodes of hypomania.

Postpartum Depression or Post-Natal Depression is clinical depression that occurs within two years of childbirth. Owing to physical, mental and emotional exhaustion combined with sleep-deprivation, motherhood can "set women up", so to speak, for clinical depression.

Premenstrual dysphoriais is a pattern of recurrent depressive symptoms tied to the menstrual cycle. The premenstrual decline in brain serotonin function is strongly correlated with the concomitant worsening of self-rated cardinal mood symptoms. Of considerable clinical importance, the recent understanding of premenstrual dysphoria as depression points directly to effective treatment with Selective serotonin reuptake inhibitor (SSRI) antidepressants. Previously, disrupting ovarian cyclicity had been the only recognized treatment. A recent review of studies of a number of SSRIs has revealed that they can effectively ameliorate symptoms of premenstrual dysphoria and may actually work best when taken only during the part of the menstrual cycle when dysphoric symptoms are evident.

What are typical depression symptoms?

If someone has the following symptoms maybe they have depression as an illness and they do not simply feel sad or low:
Pervasive low mood and tearfulness, unresponsive to and out of keeping with current circumstances.
Other mood tensions including tension, irritability and anxiety.
Insomnia (typically waking early)
Reduction in appetite, weight loss and constipation
Lack of energy and tiredness/lack of stamina
Loss of libido
Loss of concentration/forgetfulness
Feelings of low self-esteem, guilt and helplessness
Thoughts of suicide
A depressive episode is diagnosed if 5 or more of these symptoms last most of the day, nearly every day, for a period of 2 weeks or longer.