Euthymia is a normal non-depressed, reasonably positive mood. It is distinguished from hyperthymia, which refers to an extremely happy mood, and dysthymia, which refers to a depressed mood. It is a term used frequently in mental status exams.
The term is also sometimes used referring to the neutral mood (absence of a depressive or manic cycle) that some people with bipolar disorder experience with varying frequency.
References
Euthymia (medicine) Wikipedia(Text) CC BY-SA