Jealousy generally refers to the thoughts or feelings of insecurity, fear, and concern over a relative lack of possessions.
Jealousy can consist of one or more emotions such as anger, resentment, inadequacy, helplessness or disgust. In its original meaning, jealousy is distinct from envy, though the two terms have popularly become synonymous in the English language, with jealousy now also taking on the definition originally used for envy alone.
Romantic Jealousy – “a complex of thoughts, feelings, and actions that follow threats to self-esteem and/or threats to the existence or quality of the relationship when those threats are generated by the perception of a real or potential romantic attraction between one’s partner and a (perhaps imaginary) rival.” ( White, Gregory L. (1 December 1981). “A model of romantic jealousy”. Motivation and Emotion. 5 (4): 295–310. doi:10.1007/BF00992549. ISSN 0146-7239.)
Sexual Jealousy – Sexual jealousy may be triggered when a person’s significant other displays sexual interest in another person. The feeling of jealousy may be just as powerful if one partner suspects the other is guilty of infidelity. Fearing that their partner will experience sexual jealousy the person who has been unfaithful may lie about their actions in order to protect their partner. (Buunk, Bram; Hupka, Ralph B (1987). “Cross-Cultural Differences in the Elicitation of Sexual Jealousy”. The Journal of Sexual Research. 23: 12–22. doi:10.1080/00224498709551338.)
The most correct definition would be the feeling of being threatened by a rival in a romantic relationship or losing something of value to another person which happens to be yours already. (urban dictionary)
Psychology Today article about jealousy