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.
Upon delivering a letter from Doctor Jeremy Lucas to his pen pal Denobulan doctor Phlox, Ensign Hoshi Sato mentioned that the crew of Enterprise NX-01 were becoming jealous because Phlox had been receiving more letters than anyone else onboard the starship. (ENT: "Dear Doctor")
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 brief duration on the USS Voyager, Neelix felt Lieutenant Tom Paris was looking at Kes in some special manner and compared his behavior to that of a Rectilian vulture. Neelix expressed these feelings to Kes, who stated that she and Tom were "only friends", which made Neelix even more jealous because Kes had used the lieutenant's first name. (VOY: "Phage") These emotions resurfaced when Paris gave Kes a locket as a present for her birthday. Neelix denied he felt annoyed because of Paris, referring to jealousy as a "green-eyed pus hog". However, he later attempted to discuss the matter of jealousy with Chakotay. (VOY: "Twisted")