A physical state where you are more adept at tasks which are difficult when too sober or too drunk (e.g. bowling, pool, beer pong, hitting on the opposite sex)
See:
Equilibrium - a state of balance
inebriated - made drunk, intoxicate
I always start out sucking at beer pong, but there is that magic time in the night when I hit equinebrium and I just start making cups.
A person who is very knowledgeable and enthusiastic about miniature toy horses, often in times of depression, young wizards will turn to it as an escape from reality.
Harry in a spiral of depression, turns to the escape of the world of Miniature Equine Aficionada.