Bipolar disorder or manic–depressive disorder (also referred to as bipolar affective disorder or manic depression) is a psychiatric diagnosis that describes a category of mood disorders defined by the presence of one or more episodes of abnormally elevated energy levels, cognition, and mood and one or more depressive episodes. The elevated moods are clinically referred to as mania or, if milder, hypomania. Individuals who experience manic episodes also commonly experience depressive episodes or symptoms, or mixed episodes in which features of both mania and depression are present at the same time. These episodes are usually separated by periods of "normal" mood, but in some individuals, depression and mania may rapidly alternate, known as rapid cycling. Extreme manic episodes can sometimes lead to psychotic symptoms such as delusions and hallucinations. The disorder has been subdivided into bipolar I, bipolar II, cyclothymia, and other types, based on the nature and severity of mood episodes experienced; the range is often described as the bipolar spectrum.

Data from the United States on lifetime prevalence varies, but indicates a rate of around 1% for Bipolar I, 0.5–1% for Bipolar II or cyclothymia, and 2–5% for subthreshold cases meeting some, but not all, criteria. The onset of full symptoms generally occurs in late adolescence or young adulthood. Diagnosis is based on the person's self-reported experiences, as well as observed behavior. Episodes of abnormality are associated with distress and disruption, and an elevated risk of suicide, especially during depressive episodes. In some cases it can be a devastating long-lasting disorder; in others it has also been associated with creativity, goal striving and positive achievements; there is significant evidence to suggest that many people with creative talents have also suffered from some form of bipolar disorder (for more information, see list of people affected by bipolar disorder).

Genetic factors contribute substantially to the likelihood of developing bipolar disorder, and environmental factors are also implicated. Bipolar disorder is often treated with mood stabilizer medications, and sometimes other psychiatric drugs. Psychotherapy also has a role, often when there has been some recovery of stability. In serious cases in which there is a risk of harm to oneself or others involuntary commitment may be used; these cases generally involve severe manic episodes with dangerous behavior or depressive episodes with suicidal ideation. There are widespread problems with social stigma, stereotypes and prejudice against individuals with a diagnosis of bipolar disorder. People with bipolar disorder exhibiting psychotic symptoms can sometimes be misdiagnosed as having schizophrenia, another serious mental illness.

The current term "bipolar disorder" is of fairly recent origin and refers to the cycling between high and low episodes (poles). A relationship between mania and melancholia had long been observed, although the basis of the current conceptualisation can be traced back to French psychiatrists in the 1850s. The term "manic-depressive illness" or psychosis was coined by German psychiatrist Emil Kraepelin in the late nineteenth century, originally referring to all kinds of mood disorder. German psychiatrist Karl Leonhard split the classification again in 1957, employing the terms unipolar disorder (major depressive disorder) and bipolar disorder.