A mood swing is an extreme or sudden change of mood. Such changes can play a positive or a disruptive part in promoting problem solving and in producing flexible forward planning. When mood swings are severe, they may be categorized as part of a mental illness, such as bipolar disorder, where erratic and disruptive mood swings are a defining feature.

To determine mental health problems, people usually use charting with papers, interviews, or smartphone to track their mood/affect/emotion. Furthermore, mood swings do not just fluctuate between mania and depression, but in some conditions, involve anxiety.

Definitions of the terms mood swings, mood instability, affective lability, or emotional lability are commonly similar, which describe fluctuating or oscillating of mood and emotions. But each has unique characteristics that are used to describe specific phenomena or patterns of oscillation. Different from emotions or affect, mood is associated with emotional responses without knowing the reason (being unaware).

The dynamics of mood, mood patterns for long times are commonly erratic, labile or instable, also known as euthymic. Although the term of mood swing is unspecific, it may be used to describe a pattern where mood goes down from positive to negative valency immediately (without delay in baseline) at specific periods. And also generally have aperiodic patterns. This is because mood dynamics are influenced by various factors which can magnify or lessen fluctuations, such as when expectations become reality or not. Other terms for describing patterns are episodic, periodic, cyclothymia, rapid cycling, mixed states, short episodes, soft spectrum, diurnal variation, etc., although the definition of each term may be unclear.

Mood swings can happen any time at any place, varying from the microscopic to the wild oscillations of bipolar disorder, so that a continuum can be traced from normal struggles around self-esteem, through cyclothymia, up to a depressive disease. However, most people's mood swings remain in the mild to moderate range of emotional ups and downs. The duration of bipolar mood swings also varies. They may last a few hours – ultrarapid – or extend over days – ultradian: clinicians maintain that only when four continuous days of hypomania, or seven days of mania, occur, is a diagnosis of bipolar disorder justified. In such cases, mood swings can extend over several days, even weeks; these episodes may consist of rapid alternation between feelings of depression and euphoria.

There can be many different causes for mood swings. Some mood swings can be classified as normal/healthy reactions, such as grief processing, adverse effects of substances/drugs, or a result of sleep deprivation. Mood swings can also be a sign of psychiatric illnesses in the absence of external triggers or stressors.

Changes in a person's energy level, sleep patterns, self-esteem, sexual function, concentration, drug or alcohol use can be signs of an oncoming mood disorder.

Other major causes of mood swings (besides bipolar disorder and major depression) include diseases/disorders which interfere with nervous system function. Attention deficit hyperactivity disorder (ADHD), epilepsy, and autism spectrum are three such examples.