The oldest American butterfly is the Late Eocene Prodryas persephone from the Florissant Fossil Beds, approximately 34 million years old. The oldest butterflies are from the Palaeocene MoClay or Fur Formation of Denmark, approximately 55 million years old. Butterflies evolved from moths, so while the butterflies are monophyletic (forming a single clade), the moths are not. The earliest Lepidoptera fossils are of a small moth, Archaeolepis mane, of Jurassic age, around 190 million years ago (mya). A possible source of the name is the bright yellow male of the brimstone (Gonepteryx rhamni) another is that butterflies were on the wing in meadows during the spring and summer butter season while the grass was growing. The Oxford English Dictionary derives the word straightforwardly from Old English butorflēoge, butter-fly similar names in Old Dutch and Old High German show that the name is ancient. Culturally, butterflies are a popular motif in the visual and literary arts. Larvae of a few butterflies (e.g., harvesters) eat harmful insects, and a few are predators of ants, while others live as mutualists in association with ants. Some species are pests because in their larval stages they can damage domestic crops or trees other species are agents of pollination of some plants. Many butterflies are attacked by parasites or parasitoids, including wasps, protozoans, flies, and other invertebrates, or are preyed upon by other organisms. Some, like the monarch and the painted lady, migrate over long distances. Butterflies are often polymorphic, and many species make use of camouflage, mimicry and aposematism to evade their predators.

Some butterflies, especially in the tropics, have several generations in a year, while others have a single generation, and a few in cold locations may take several years to pass through their entire life cycle. When metamorphosis is complete, the pupal skin splits, the adult insect climbs out, and after its wings have expanded and dried, it flies off. The caterpillars grow, sometimes very rapidly, and when fully developed, pupate in a chrysalis. Winged adults lay eggs on the food plant on which their larvae, known as caterpillars, will feed. Butterflies have the typical four-stage insect life cycle. Butterfly fossils date to the Paleocene, about 56 million years ago. The group comprises the large superfamily Papilionoidea, which contains at least one former group, the skippers (formerly the superfamily Hesperioidea), and the most recent analyses suggest it also contains the moth-butterflies (formerly the superfamily Hedyloidea). Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths.

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