Enthusiasm, n.
Etymology: [Gr. enthousiasme. See Entheal, Theism.]
1. Inspiration as if by a divine or superhuman power; ecstasy; hence, a conceit of divine possession and revelation, or of being directly subject to some divine impulse. Enthusiasm is founded neither on reason nor divine revelation, but rises from the conceits of a warmed or overweening imagination. Locke.
2. A state of impassioned emotion; transport; elevation of fancy; exaltation of soul; as, the poetry of enthusiasm. Resolutions adopted in enthusiasm are often repented of when excitement has been succeeded by the wearing duties of hard everyday routine. Froude. Exhibiting the seeming contradiction of susceptibility to enthusiasm and calculating shrewdness. Bancroft.
3. Enkindled and kindling fervor of soul; strong excitement of feeling on behalf of a cause or a subject; ardent and imaginative zeal or interest; as, he engaged in his profession with enthusiasm. Nothing great was ever achieved without enthusiasm. Emerson.
4. Lively manifestation of joy or zeal. Philip was greeted with a tumultuous enthusiasm. Prescott.
Shades of meaning
The old meaning of enthusiasm implies a pseudo-inspiration, an almost frantic extravagance in behalf of something supposed to be an expression of the divine will. This sense remains as the controlling one in the kindred noun enthusiast. Enthusiasm has now chiefly the meaning of an earnest and commendable devotion, an intense and eager interest. Against the hindrances of the world, nothing great and good can be carried without a certain fervor, intensity, and vehemence; these joined with faith, courage, and hopefulness make enthusiasm. Zeal is burning earnestness, always tending to vigorous action with all the devotion of enthusiasm, tho often without its hopefulness. Compare eager.