The word stems from the French jalousie, formed from jaloux (jealous), and further from Low Latin zelosus (full of zeal), in turn from the Greek word ζήλος (zēlos), sometimes "jealousy", but more often in a positive sense "emulation, ardour, zeal"  (with a root connoting "to boil, ferment"; or "yeast").
The color green is often associated with jealousy and envy, from which the expressions "green with envy", and "green-eyed monster" are derived.
People do not express jealousy through a single emotion or a single behavior. They instead express jealousy through diverse emotions and behaviors, which makes it difficult to form a scientific definition of jealousy. Scientists still do not have a universally agreed upon definition of jealousy. They instead define jealousy in their own words, as illustrated by the following examples:
- "Romantic jealousy is here defined as a complex of thoughts, feelings, and actions which 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 attraction between one's partner and a (perhaps imaginary) rival." (White, 1981, p. 24)
- "Jealousy, then, is any aversive reaction that occurs as the result of a partner's extradyadic relationship that is real, imagined, or considered likely to occur." (Bringle & Buunk, 1991, page 135)
- "Jealousy is conceptualized as a cognitive, emotional, and behavioral response to a relationship threat. In the case of sexual jealousy, this threat emanates from knowing or suspecting that one's partner has had (or desires to have) sexual activity with a third party. In the case of emotional jealousy, an individual feels threatened by her or his partner's emotional involvement with and/or love for a third party." (Guerrero, Spitzberg, & Yoshimura, 2004, page 311)
- "Jealousy is defined as a protective reaction to a perceived threat to a valued relationship, arising from a situation in which the partner's involvement with an activity and/or another person is contrary to the jealous person's definition of their relationship." (Bevan, 2004, page 195)
- "Jealousy is triggered by the threat of separation from, or loss of, a romantic partner, when that threat is attributed to the possibility of the partner's romantic interest in another person." (Sharpteen & Kirkpatrick, 1997, page 628)
These definitions of jealousy share two basic themes. First, all the definitions imply a triad composed of a jealous individual, a partner, and a third party rival. Jealousy typically involves three people. Second, all the definitions describe jealousy as a reaction to feeling threatened. Jealous reactions typically involve aversive emotions and/or protective behaviors. These themes form the essential meaning of jealousy in most scientific studies.
Popular culture uses the word jealousy as a synonym for envy. Many dictionary definitions include a reference to envy or envious feelings. In fact, the overlapping use of jealousy and envy has a long history.
The terms are used indiscriminately in such popular 'feelgood' books as Nancy Friday's Jealousy, where the expression 'jealousy' applies to a broad range of passions, from envy to lust and greed. While this kind of usage blurs the boundaries between categories that are intellectually valuable and psychologically justifiable, such confusion is understandable in that historical explorations of the term indicate that these boundaries have long posed problems. Margot Grzywacz's fascinating etymological survey of the word in Romance and Germanic languages asserts, indeed, that the concept was one of those that proved to be the most difficult to express in language and was therefore among the last to find an unambiguous term. Classical Latin used invidia, without strictly differentiating between envy and jealousy. It was not until the postclassical era that Latin borrowed the late and poetic Greek word zelotypia and the associated adjective zelosus. It is from this adjective that are derived French jaloux, Provençal gelos, Italian geloso, and Spanish celoso. (Lloyd, 1995, page 4)
Perhaps the overlapping use of jealousy and envy occurs because people can experience both at the same time. A person may envy the characteristics or possessions of someone who also happens to be a romantic rival. In fact, one may even interpret romantic jealousy as a form of envy. A jealous person may envy the affection that his or her partner gives to a rival — affection the jealous person feels entitled to himself or herself. People often use the word jealousy as a broad label that applies to both experiences of jealousy and experiences of envy.
Although popular culture often uses jealousy and envy as synonyms, modern philosophers and psychologists have argued for conceptual distinctions between jealousy and envy. For example, philosopher John Rawls distinguishes between jealousy and envy on the ground that jealousy involves the wish to keep what one has, and envy the wish to get what one does not have. Thus, a child is jealous of her parents' attention to a sibling, but envious of her friend's new bicycle. Psychologists Laura Guerrero and Peter Andersen have proposed the same distinction. They claim the jealous person "perceives that he or she possesses a valued relationship, but is in danger of losing it or at least of having it altered in an undesirable manner," whereas the envious person "does not possess a valued commodity, but wishes to possess it." Gerrod Parrot draws attention to the distinct thoughts and feelings that occur in jealousy and envy.
The experience of jealousy involves:
The experience of envy involves:
Parrot acknowledges that people can experience envy and jealousy at the same time. Feelings of envy about a rival can even intensify the experience of jealousy. Still, the differences between envy and jealousy in terms of thoughts and feelings justify their distinction in philosophy and science.