Bipolar disorder, also known as manic-depressive illness, is a mental health disorder characterized by extreme mood swings that alternate between periods of mania and depression. Individuals with bipolar disorder may experience periods of elevated or irritable mood, known as mania or hypomania, as well as periods of depression.

Symptoms of bipolar disorder can vary in severity and may include:

1. Manic or hypomanic episodes, characterized by high energy levels, impulsivity, racing thoughts, increased self-esteem, decreased need for sleep, and sometimes psychotic symptoms such as hallucinations or delusions
2. Depressive episodes, characterized by low mood, feelings of worthlessness, lack of energy, changes in appetite or sleep patterns, and thoughts of suicide or self-harm
3. Mixed episodes, characterized by symptoms of both mania and depression occurring at the same time

Bipolar disorder can be caused by a combination of genetic, biological, and environmental factors. It can affect people of all ages, but typically develops in the late teenage years or early adulthood.