Quite Interesting: Difference Between 'Jealousy' and 'Envy'

According to Stanford University's Encyclopedia of Philosophy, the terms 'jealousy' and 'envy,' though often conflated, are not meant to be used interchangeably. This article says that jealousy involves three parties, whereas envy involves only two. For example, a lover can be described as being "jealous" when his or her mate is showing another person affection:

the jealous person's real locus of concern is the beloved—the person whose affection he is losing or fears losing—not his rival. Whereas envy is a two party relation, with a third relatum that is a good (albeit a good that could be a particular person's affections); and the envious person's locus of concern is the rival. Hence, even if the good that the rival has is the affection of another person, there is a difference between envy and jealousy.

In other words, where jealousy is concerned, said feeling is not dependent on the rival, but on the flirtations of the lover, whose affections the jealous person is afraid of losing. When it comes to envy, however, the feeling is completely rival-dependent (the person is envious of something that the rival possesses, and the situation does not involve competition for another individual).