A mild bipolar disorder that persists over a long time WordNet 3.0 & Open English WordNet

Cyclothymia, also known as cyclothymic disorder, psychothemia / psychothymia, bipolar III, affective personality disorder and cyclothymic personality disorder, is a mental and behavioural disorder that involves numerous periods of symptoms of depression and periods of symptoms of elevated mood. Wikipedia

Persistent instability of mood that involves recurrent hypomanic and dysthymic episodes, but no full manic episodes or full major depressive episodes Wikidata

A chronic mental disturbance characterized by mood swings and depression. Wiktionary

Chronic mental disturbance characterized by mood swings and depression. Wiktionary (translation)