Euthymia just means to be in a calm and tranquil mood.
There is a generally accepted medical definition that is frequently used with bipolar and depressive disorders to describe a stable mood (at least in the UK, I assume it is in other English speaking countries).
Given the community we are in I think it’s being used suitably here 😊
Ennui is more despair*. The center is half way between mania and depression, and half way between happiness and sadness. It’s a state of calm and tranquility. Not happy or sad, but not manic or depressed.Edit:
*Sorry, don’t think I could have been more unclear with that if I tried. What I meant to say was “Ennui is like Euthymia, but with a touch of despair. It’s more about feeling a little down and listless.”