A butterfly was a diurnal insect characterized by a slender body, knobbed antennae, and four broad, conspicuously colored wings.
The holographic baseball park program where Captain Benjamin Sisko trained the members of his baseball team, the Niners, in 2375, contained a simulation of a white butterfly that flew in the infield behind the pitcher's mound. (DS9: "Take Me Out to the Holosuite")
When in 2376 Icheb was preparing to meet his parents for the first time since he had been assimilated, he told The Doctor that he has a strange feeling in his stomach. The Doctor jokingly replied that he's got butterflies in there. Icheb, never having heard that expression, told The Doctor that he in fact had "never assimilated butterflies before". (VOY: "Child's Play")