How do you define bipolar disorder?

See DSM criteria. You can look up the exact criteria used in research studies to learn more about it by searching online. The phrase "bipolar" is often overused, cheapening a formal diagnosis. It encompasses large swings in mood/energy/speed of thoughts/behavior which may last days/weeks at a time. See your local psychiatrist or primary care doctor for screening. Meds are most helpful but counseling can help.
Bipolar. Is a mood disorder characterized by mood swings from mania (or mixed mania with depression) to depression. To have bipolar, there must have been at least one episode of mania or mixed presentation.