Jealousy was an emotion that refered 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 starship Voyager, Neelix felt Tom Paris was looking at Kes in some special manner and compared his behavior to that of a rectilian vulture (a predator). 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")