means Intense, energetic enthusiasm or fervor in pursuit of a cause, belief, or goal.

from From Latin 'zelus,' borrowed straight from Greek 'zēlos' — a word that meant ardor, eager rivalry, even jealousy (its close cousin). The Greeks felt the heat in it: to be full of zēlos was to burn with passionate striving. It traveled through Late Latin into English in the late Middle Ages, often arriving wrapped in religious garb, the fire of the faithful.