Zeal, n.
Etymology: [F. zèle; cf. Pg. & It. zelo, Sp. zelo, celo; from L. zelus, Gr. Yeast, Jealous.]
1. Passionate ardor in the pursuit of anything; eagerness in favor of a person or cause; ardent and active interest; engagedness; enthusiasm; fervor. "Ambition varnished o'er with zeal." Milton. "Zeal, the blind conductor of the will." Dryden. "Zeal's never-dying fire." Keble. I bear them record that they have a zeal of God, but not according to knowledge. Rom. x. 2. A zeal for liberty is sometimes an eagerness to subvert with little care what shall be established. Johnson.
2. A zealot. [Obs.] B. Jonson.
1 other Webster entry under this spelling
Zeal, v. i.
To be zealous. [Obs. & R.] Bacon.
Shades of meaning
Fernald has no entry of its own for zeal; he lists it under enthusiasm.
Where he writes about it:
Zeal is burning earnestness, always tending to vigorous action with all the devotion of enthusiasm, tho often without its hopefulness. — from enthusiasm