Cyclothymia is a bipolar variant that involves (1) recurrent episodes of hypomania that don’t meet criteria for a manic or mixed episode and (2) recurrent bouts of depressive symptoms that don’t meet criteria for a major depression. It’s helpful to think in terms of mood symptoms having amplitude. In cyclothymia, neither the highs nor the lows ever make it to the level of a manic, mixed, or depressed episode.
People with Cyclothymia cycle through their moods rapidly, and part of the diagnostic criteria is that they go for at least two years without a two-month symptom-free stretch. Many people who are first diagnosed with cyclothymia in their late teens and twenties will eventually progress to either bipolar I or bipolar II.
In older texts, cyclothymia was often referred to as cyclothymic neurosis, and was believed to be more in the realm of a personality disorder. This belief changed partially in response to the observation that many of these people do progress to have a more classic bipolar presentation and the realization that mood-stabilizing medication can be helpful in diminishing the mood swings associated with cyclothymia.