Boredom was an emotion caused by lack of mental stimulation.
In 2364, Captain Jean-Luc Picard asked Data if Commander William T. Riker was alive, to which Data answered, "I would guess that death is no longer sufficient entertainment to alleviate its boredom." (TNG: "Skin Of Evil")
Elias Giger's theory of cellular ennui posited that death by "natural causes" was nothing more than the result of cellular boredom. As he explained it, the cells of the body kept doing the same boring routine, eventually simply saying "that's it," and the unwitting victim becoming literally bored to death. As such, he created the cellular regeneration and entertainment chamber to keep the cells entertained. (DS9: "In the Cards")