Ask ten different people and you might get ten different views on how the emotions of envy and jealousy are distinct.
It turns out that psychologists agree on a fairly straightforward distinction.
Envy occurs when we lack a desired attribute enjoyed by another.
Jealousy occurs when something we already possess (usually a special relationship) is threatened by a third person
And so envy is a two-person situation whereas jealousy is a three-person situation. Envy is a reaction to lacking something. Jealousy is a reaction to the threat of losing something (usually someone).
This seems straightforward, and so why the confusion?
One problem is an unfortunate sematic ambiguity with the word “jealousy” (but NOT with the word “envy”). If you ask people to describe a situation in which they felt jealous, they are as likely to describe an experience of envy (e.g., "I wished I had my friend's good looks") as of jealousy (e.g., "my girlfriend danced with an attractive guy"). Naturally, this creates a sense that jealousy and envy are very similar—even though they are actually quite different.
Therefore, when someone says, “I’m feeling jealous,” you don’t know whether he or she is experiencing an envy situation or a jealousy situation—unless more context is provided (e.g., “I felt jealous when I saw my girlfriend dancing with the attractive guy”).
The second problem is that envy and jealousy often travel together. What kind of rival to your partner’s affections is likely to create jealousy? It is the rival with characteristics that you are also likely to envy—that is, the attractive rival.
This means that when you are feeling jealous, you are often feeling envious as well.
And yet envy and jealousy are not the same emotions. Envy, as unpleasant as it can be, usually doesn’t contain a sense of betrayal and resultant outrage, for example. Jealousy need not contain an acute sense of inferiority (if the rival is not enviable).
Here is the envious Cassius as he laments Caesar’s advantages:
"Why, man, he doth bestride the narrow world
Like a Colossus: and we petty men
Walk under his huge legs, and peep about
To find ourselves dishonorable graves."
And here is the jealous Othello as he rages against Desdemona:
"All my fond love thus to I blow to heaven.
Arise, black vengeance, from the hollow hell!"
Notice how much Cassius focuses on his feelings of inferiority and how much Othello is enraged and vengeful. Both emotions lead to murder, but because they arise from different situations, they are qualitatively distinct in felt experience. Cassius does not feel personally betrayed by Caesar; Othello does not feel inferior to person whom he believe Desdemona has betrayed him for.
One thing for sure is that there is hardly a more intense, unpleasant emotional brew that the reaction caused by seeing your loved one show interest in an enviable rival. The blend of jealousy and envy is a debiltating kick in the emotional solar plexus.
See the book, The Joy of Pain: Schadenfreude and the Dark Side of Human Nature, for more on these and related topics, especially on envy.