The tone level of 4.0 on the Tone Scale. Enthusiasm means a person feels very cheerful yet also maintains rationality. It’s not a high manic state. A person at the level of Enthusiasm has a sense of vitality. They are eager to get involved in activities important to them. Stably reaching this point also means they are able to access their naturally optimum state of intelligence.
(Dianetics: The Original Thesis, Chapter The Character of Engrams) (Notes on The Lectures, (There are two chapters called The Tone Scale. While both are informative about the tone scale, the second one is where info for this definition came from.)