in the context of kava I'm fairly specific in what I mean by "euphoria".
It is a kind of a rush of feeling of well-being, both mental and physical, that kind of washes over you, fairly suddenly, and envelopes you, sometimes all at one go, sometimes in waves. It can last anywhere from 1 - 20 minutes. It is quite pronounced. For me sometimes swirling colours are involved, an involuntary smile response, a slight feeling of delicious melting right behind my eyes, warm throbbing sensations at the top of my head that work their way down, followed soon by a feeling of peacefulness and floating. It definitely feels happy and even joyous. Typically it happens at the beginning of a kava experience and then subsides as the more relaxed, mellow part of the kava kicks in (I believe the euphoria effect is traceable to 2 or 3 specific kavalactones that are active frst).
Some euphorias are only on the first shell while others, like Damu, occur with each shell, for me, anyway.
I'm finding with my new found enjoyment of triple balling that this prep tends to bring out any latent euphoric potential in a number of grinds, more so than any other prep.
The closest thing I have found elsewhere is the runners high. Not the general feeling of well-being that many feel at the end of a nice run and call a runners high, but a specific reaction to oxygen intake after a very hard run of 10 miles or more at a fast pace, where suddenly colours become brighter for anywhere from a minute or two, which is most common, but up to ten minutes or so. Accompanied, obviously, by a feeling of intense joy and wellness. I find that 90% of the time or more when people say "runner's high" they aren't referring to this, probably because they have never experienced it. It's definitely something else, quite distinct.