Intense, fervent, energetic devotion to a cause or pursuit.
Intense, fervent, energetic devotion to a cause or pursuit — drive that carries enthusiasm into action.
The high-energy, action-driving member: it “implies energetic and unflagging pursuit of an aim or devotion to a cause.” More intense, sustained, and goal-directed than enthusiasm (keen interest), and it readily collocates with excess (“fanatical zeal”), carrying a faintly cautionary edge enthusiasm lacks. Stronger and more single-minded than eagerness or interest; unlike longing or hope it is not about awaiting something but strenuously pursuing it. Etymologically kin to jealous (both from Greek zēlos), which is why it can shade toward possessive intensity.
From Greek zēlos “ardor, eager rivalry” (also “jealousy”), via Late Latin zelus into English in the late 14c. A Church word; the same root gives jealous and zealot.
From its 14c. entry the core sense — passionate ardor in pursuit of an aim — has been stable (it could from the start also apply to wrath or vengeance). Its kinship with jealousy (same Greek root) and the disparaging “zealot” (a fanatical enthusiast, from the 1630s) are the notable threads. No reliable recent-generation shift in the core sense.