Jealousy was an emotion that referred to the thoughts, feelings, and behaviors that occurred when a person believed a valued relationship was being threatened by a rival.
In order to free himself and his crew from the Kelvans, Captain James T. Kirk aroused feelings of jealousy in the as-yet emotionally immature aliens – Kirk caused Rojan to become jealous by seducing Kelinda. (TOS: "By Any Other Name")
After only a few time on the USS Voyager, Neelix felt Tom Paris was looking at Kes in some special manner and compared his behavior to that of a rectilian vulture. He told it Kes, who stated Tom and her were "only friends", which made Neelix even more jealous for the use of the Lieutenant's first name. (VOY: "Phage") These emotions resurfaced when Paris gave Kes a locket as a present for her birthday. He denied he felt annoyed because of Paris, referring to jealousy as "green-eyed pus hog". However he later attempted to discuss the matter of jealousy with Chakotay. (VOY: "Twisted")