Cyclothymic is a word that describes a mental state characterized by alternating periods of happiness and depression. It is often used to describe the mood swings of people who suffer from bipolar disorder. However, there are many synonyms for this word that can be used to describe similar mental states. Examples of these synonyms include fluctuating, mercurial, volatile, ever-changing, and temperamental. Other synonyms include erratic, unpredictable, and capricious. Each of these words can be used to describe someone who experiences abrupt changes in mood, behavior or personality, which may be difficult for others to anticipate or understand.