persistent instability of mood that involves recurrent hypomanic and dysthymic episodes, but no full manic episodes or full major depressive episodes
Cyclothymia is a mental illness characterized by mood swings - a mood disorder. The disease is a milder version of bipolar disorder. The mood swings are usually less extreme. Mood swings can be spontaneous, or triggered by an event.
Cyclothymia is also an old name for bipolar disorder.