What is the difference between envy and jealousy, biblically speaking?

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 Ps. 69:9, where the LXX (68:9) 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. Afterwards he says he was jealous (ζηλόω, the verb) over them with a godly jealousy (ζῆ.) 2 Cor. 11:2; but in 2 Cor. 12:20, he uses the same word for the 'envyings' or 'jealousies' he feared he might find among them.

"Love does not envy," though jealousy is in its very nature.5 God, who is the source and final object of His own desires, working as He wills, can envy none. Nor can that love which has its origin and hope in Him. Love covets earnestly,6 but not for its own sake. It is jealous over those who are its care.7 But because it knows God, it can neither crave addition to His gift unspeakable which it already has in Christ, nor can it look angrily on any who in position may seem higher than itself; for in God's light only it sees light. As love is of God, so envy is distinctively of Satan. It was a prime element in the original transgression, and was the spring of Cain's deadly hatred to his brother. And when the appointed hour came for darkness to sit in judgment upon light, it was through envy that the enemy prevailed.8 In God's presence it can find no place, though it hovers often at the threshold of the sanctuary. Moses was meeker than his servant, because Moses spake with God, while Joshua only spake with Moses.9 When the disciples wrangled for supremacy, the Master's back was turned.1 In His presence emulation takes a downward course.

Jealousy is cruel as the grave. How pitilessly cruel is the grave. It swallows up the young, the loved, the fairest, and the brightest. It knows no pity, and so jealousy would deal without pity against all that would come between the Bridegroom and His bride. Christ must be supreme: "He that loves father or mother more than me is not worthy of Me," and therefore the Lord can say, "If any man come to Me, and hate not his father, and mother, and wife and children, and sister, yea, and his own life also, he cannot be my disciple." To "hate" has the sound of cruelty, but it is the cruelty of jealous love that brooks no rival. Almost universally men speak of jealousy in an evil sense, Scripture hardly ever so. It speaks even of a "godly jealousy." The Apostle can say of believers, "I am jealous over you with a godly jealousy" for, says he, "I have espoused you to one husband, that I may present you as a chaste virgin to Christ" (2 Cor. 11:2). His love to Christ, and his love to the saints made him jealous lest anyone, or anything, should come between them and Christ. He had no pity for any who, by false doctrines, would beguile the saints from Christ. If an apostle or an angel from heaven preach any other gospel let him be accursed. This was the cruelty of jealous love.

Love strong as death, and jealousy cruel as the grave are found together. One is the outcome of the other. Love and jealousy may be found in measure in all men. But it is only love strong as death that calls forth jealousy cruel as the grave.

qana, 'to be inflamed.' The warm affection that cannot bear to see its loved one enticed by another, as a man is jealous of his wife, Num. 5:14; as Paul felt for the Corinthian saints, 2 Cor. 11:2; and as God regarded the people and the land which He had chosen, and upon which He had placed His name. Ps. 79:5; Ezek. 39:25; Joel 2:18; Zech. 1:14; Zech. 8:2. "Jehovah, whose name is jealous, is a jealous God." Ex. 34:14.

Then we have envyings. "Who is able to stand before envy?" Prov. 27:4. This is another very common fault; perhaps much more common than we realize. How often we envy the wealth, or the ability, or the gift, or the holiness of another. This spirit of envy raises in my heart a spirit of enmity against my brother, and a long list of works of the flesh may result; all started from envy. It may be that the brother I am envying has not the least idea that he has been the cause of so much evil in my life; yet the fault was not with him, but entirely with myself, as I allowed the flesh to act in envy.

Then come eight bad works towards my brethren. First, enmities. How easy it is to have a spirit of enmity against a brother. He may have wronged us, and we do not forgive; after the enmities come quarrelling. The enmities we may have kept in our heart, but very soon it comes out in an open quarrel. The fault, we think, is all on his side; but when we were children and we quarrelled together, my mother always told us: "It takes two to make a quarrel." We may be certain if we are mixed up in a quarrel, it is because we have yielded to the flesh. Only by pride cometh contention! (Prov. 13:10.) Then comes jealousy, one of the most common things in the heart of man. Then outbursts of anger. We call it "quick-tempered" or "short-tempered," but it is one of the works of the flesh. In 2 Peter 3:9 we read that the Lord is "long-tempered to us-ward." Then self-seeking. In Philippians 2:21 we read: "All seek their own, not the things which are Jesus Christ's." Nearly all the Christians, even of Paul's day, had fallen under the power of this terrible work of the flesh. There was one young man who was exempt, and that was Timothy; so we might see that there is a way not to put into effect even this work of the flesh.

The vainglorious man will ever be an envious man — jealous of every one that is more spiritual or more gifted than himself. And jealousy expresses itself in malice, and malice ends in strife (James 3:14-16).

Moreover, we do well to remember that envy is always the confession of inferiority. To envy a man with a big income is to admit that one's own is smaller. In the same way, to be jealous of a man with gift is to confess that one's own is an inferior gift.

Envy involves both personal selfishness and ill-feeling toward another. Strife therefore accompanies it. This in turn overthrows all proper equilibrium: disorder prevails, and leaves the door open for "every evil work." It is by this means that Satanic activity thrives.

Suppose by way of illustration that envious feelings, or a hard and unforgiving spirit, is allowed or cultivated in the heart of any believer towards any of his brethren; or suppose one brother becomes jealous of another because he seems to have more honour than himself,—these are bitter roots from which will spring “hatred, variance, emulations, wrath, and strife”—works of the flesh, and most deadly in their effect wherever they show themselves. How often have we seen happy Christian companies blighted and broken by “bitterness, wrath, anger, clamour, and evil speaking;” or as a result of one or more who ought to have served all in love, lording it over the Lord's heritage. Our association with all the saints of God is a divinely-formed association, and we must not, we cannot, ignore it. The spiritual prosperity of one is a help to all, and the works of the flesh in any means harm to all. We can no longer live as though we alone are affected by our living, “for if one member suffer all the members suffer with it.”