Both ζῆλος and φθόνος are translated 'envy,' but they are not used indiscriminately. Ζῆλος is sometimes used in a good and sometimes in a bad sense, whereas φθόνος is perhaps always used in a bad sense in the New Testament.
In Gal. 5:20, 21, both words are among the "works of the flesh," as ‘emulations' or ‘jealousies' (ζῆ.) and ‘envyings ' (φθ.).
Ζῆλος is used of the Lord in "The ‘zeal' of thine house hath eaten me up," John 2:17: a quotation from Psalm 69:9 (68:10), where the LXX has the same word. Paul speaks to the Corinthian saints of the ‘zeal' ('fervent mind' A. V.) they had for him, and the ‘zeal' they manifested in clearing themselves from the evil amongst them. 2 Cor. 7:7, 11. Afterward he says he was jealous (ζηλόω, the verb) over them with a godly jealousy (ζῆ.), chap. 11:2; but in chap. 12:20, he uses the same word for the ‘envyings' or ‘jealousies' he feared he might find among them.
Above it is said, ‘perhaps' φθόνος always has a bad sense in the N. T. because of Jas. 4:5. This is a difficult passage, and has been variously translated. Apparently the A. V. and R. V. assume the ‘spirit' to be the human spirit, which naturally lusteth to envy; but others believe the spirit to be the Holy Spirit, and if so, may not the sense be, as given by Bengel, "Does the Spirit, which has taken up his dwelling in us, lust enviously?" This latter interpretation is in all probability the right one.