euthymia

noun

eu·​thy·​mia yü-ˈthī-mē-ə 

: a normal, tranquil mental state or mood

specifically : a stable mental state or mood in those affected with bipolar disorder that is neither manic nor depressive

… there were no significant differences among groups in the rates of durable recovery, defined as 8 consecutive weeks of euthymia without a switch to mania or hypomania.

—Mark A. Frye, The New England Journal of Medicine

euthymic

-mik 

adjective

the euthymic phase of bipolar disorder