A butterfly is an insect of the order Lepidoptera, and belongs to one of the superfamilies Hesperioidea (the skippers) or Papilionoidea (all other butterflies). Some authors have also suggested the inclusion of the superfamily Hedyloidea, the American butterfly moths. They are notable for their unusual life cycle with a larval caterpillar stage, an inactive pupal stage and a spectacular metamorphosis into a familiar and colourful winged adult form. The diverse patterns formed by their brightly coloured wings and their erratic-yet-graceful flight have made butterfly watching a popular hobby.

The Old English word for butterfly was buttorfleoge apparently because butterflies were thought to steal milk. A similar word occurs in Dutch and German originating from the same belief. This is believed to have led to the evolution of its present name form - butterfly.
An alternative folk etymology, prevalent in Great Britain, is that it originated as a contraction of butter-coloured fly referring to the Brimstone Butterfly Gonepteryx rhamni, often the first butterfly of spring. Another such view is that the word butterfly came from a metathesis of "flutterby". wikipedia