Jealousy and envy are often used interchangeably. They're different.
Envy is about wanting what someone else has — a quality, a position, a relationship, an achievement.
Jealousy is about fearing the loss of something you already have — usually to a rival.
See Envy for a full exploration of that emotion.
What Jealousy Is Pointing At
Jealousy points toward insecurity — what you're afraid of losing and why. It's frequently misdirected: the actual issue is usually not the perceived rival but an underlying doubt about your own worth or the strength of the bond.
Both emotions become destructive when acted on impulsively or suppressed entirely. Examined honestly, jealousy is one of the more revealing feelings available — it shows you exactly where your sense of security is fragile.
The Question Worth Asking
When jealousy surfaces, the useful question is not who is threatening what I have but why do I feel this could be taken? The answer almost always lives in self-doubt rather than in any external threat.