How "envy" is different than "jealousy"

What is the difference between envy and jealousy? This question has been pondered by authors in the fields of psychology, history, and lexicography, and they often arrive at different answers.  

Lexicographer Bryan Garner asserts that jealousy “is properly restricted to contexts involving emotional rivalry,” while envy is “resentful contemplation of a more fortunate person.” Social psychologist Brené Brown contrasts them in a different way: Envy is “when we want something that another person has,” and jealousy is “when we fear losing a relationship ... that we already have.” 

I am with Merriam-Webster, which concludes that it is impractical to try to pin them down. The two words have acquired many connotations since they first appeared in the 13th century, and “many educated people use them interchangeably.” It’s common to comment “I’m so jealous!” in response to a friend’s pictures from a fabulous vacation, for example, though according to the previous definitions you would properly be envious.

Attempts to distinguish them are perhaps influenced by their etymologies. Envy comes most directly from the French envie (“want”). “J’ai envie d’une glace” is “I want an ice cream,” not “I envy that double scoop of vanilla.” The French envie derives in turn from invidia (in- + videre, “to look back at”) which referred to the “evil eye” in ancient Rome. It makes certain etymological sense, then, to cast envy as “I want what I don’t have.”

Jealous, rather surprisingly, is the twin of zealous. Both come from the Greek zēlos, which in ancient Athens was a generally positive term, meaning “eager rivalry, fervor.” In English, jealous, like zealous, was originally an indication of the strength of an emotion, not its content – “vehement in feeling, as in wrath, desire, or devotion,” according to the Oxford English Dictionary. In the Middle Ages and Renaissance, people who were industrious at work, ardent in pursuing relationships, and vigilant about defending their honor could all be called jealous. Merriam-Webster notes that on the flip side, zealous “occasionally was used in biblical writing to refer to a quality of apprehensiveness or jealousy of another.” In the 15th century, these words diverged. Zealous got the positive sense, “warmly engaged or ardent on behalf of someone or something,” while jealous got the negative, “intolerant of rivalry or unfaithfulness” and “vigilant in guarding a possession.” (Zealous is developing a negative connotation today, too, carrying a sense of “excessive feeling.”)

Those who want to clearly separate envy from jealousy are zealous in their nitpicking.