From G2204 (zeō, to boil). Intense emotion — positive (zeal, devotion) or negative (jealousy, envy). Context determines which.
Zēlos is morally ambivalent. Positive: Jesus cleansed the temple because 'the zeal of thine house hath eaten me up' (John 2:17). Negative: James warns 'where envying and strife is, there is confusion' (James 3:16). Passion directed at God's glory is holy fire; directed at self-interest, consuming flame. Passion must be governed by truth.