Anxious, resentful fear of losing something you have.
Anxious, resentful fear of losing something you have — especially a relationship or affection — to a rival.
The fear and resentment of losing what one has to a rival — protectiveness over one's own attachments (the conventional three-person situation: you, the loved one, the rival), versus envy's covetousness of what another has (two-person). It is the word used in romance (“the green-eyed monster”), and “jealousy is enmity prompted by fear.” Common usage often blurs jealous and envious.
Suspicious resentment at the threat of losing a loved one's affection to a rival.
The dominant sense, attested from c. 1200; Shakespeare's “green-eyed monster.”
Fierce, watchful guarding of something cherished — “a jealous God,” guarding freedoms “with jealousy.”
A relic of the word's positive origin in zeal; jealous and zealous are doublets from the same root.
c. 1200, from Old French jalousie, from jalos “zealous; jealous,” from Late Latin zelosus, from zelus “zeal,” from Greek zēlos. So jealousy shares its root directly with zeal and zealous.
The zeal connection is the key thread: in Middle English jealousy could mean “solicitude, regard” and even “zeal, devotion” (the connecting notion being “watchfulness”). The surviving “zealous vigilance” sense is a relic of that positive origin. No reliable recent-generation shift.