Lepidoptera (/ˌlɛpɪˈdɒptərə/ LEP-ih-DOP-tər-ə) or lepidopterans are an order of winged insects which include butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organisms, making it the second largest insect order (behind Coleoptera) with 126 families and 46 superfamilies, and one of the most widespread and widely recognizable insect orders in the world.

Lepidopteran species are characterized by more than three derived features. The most apparent is the presence of scales that cover the bodies, large triangular wings, and a proboscis for siphoning nectars. The scales are modified, flattened "hairs", and give butterflies and moths their wide variety of colors and patterns. Almost all species have some form of membranous wings, except for a few that have reduced wings or are wingless. Mating and the laying of eggs is normally performed near or on host plants for the larvae. Like most other insects, butterflies and moths are holometabolous, meaning they undergo complete metamorphosis. The larvae are commonly called caterpillars, and are completely different from their adult moth or butterfly forms, having a cylindrical body with a well-developed head, mandible mouth parts, three pairs of thoracic legs and from none up to five pairs of prolegs. As they grow, these larvae change in appearance, going through a series of stages called instars. Once fully matured, the larva develops into a pupa. A few butterflies and many moth species spin a silk casing or cocoon for protection prior to pupating, while others do not, instead going underground. A butterfly pupa, called a chrysalis, has a hard skin, usually with no cocoon. Once the pupa has completed its metamorphosis, a sexually mature adult emerges.

Lepidopterans first appeared in fossil record in the Triassic-Jurassic boundary and have coevolved with flowering plants since the angiosperm boom in the Middle/Late Cretaceous. They show many variations of the basic body structure that have evolved to gain advantages in lifestyle and distribution. Recent estimates suggest the order may have more species than earlier thought, and is among the five most species-rich orders (each with over 100,000 species) along with Coleoptera (beetles), Diptera (flies), Hymenoptera (ants, bees, wasps and sawflies) and Hemiptera (cicadas, aphids and other true bugs). They have, over millions of years, evolved a wide range of wing patterns and coloration ranging from drab moths akin to the related order Trichoptera, to the brightly colored and complex-patterned butterflies. Accordingly, this is the most recognized and popular of insect orders with many people involved in the observation, study, collection, rearing of, and commerce in these insects. A person who collects or studies this order is referred to as a lepidopterist.

Butterflies and moths are mostly herbivorous (folivorous) as caterpillars and nectarivorous as adults. They play an important role in the natural ecosystem as pollinators and serve as primary consumers in the food chain; conversely, their larvae (caterpillars) are considered very problematic to vegetation in agriculture, as they consume large quantity of plant matter (mostly foliage) to sustain growth. In many species, the female may produce from 200 to 600 eggs, while in others, the number may approach 30,000 eggs in one day. The caterpillars hatching from these eggs can cause significant damage to crops within a very short period of time. Many moth and butterfly species are of economic interest by virtue of their role as pollinators, the silk in their cocoon, or for extermination as pest species.

The term Lepidoptera was used in 1746 by Carl Linnaeus in his Fauna Svecica. The word is derived from Greek λεπίς lepís, gen. λεπίδος lepídos ("scale") and πτερόν ("wing"). Sometimes, the term Rhopalocera is used for the clade of all butterfly species, derived from the Ancient Greek ῥόπαλον (rhopalon) and κέρας (keras) meaning "club" and "horn", respectively, coming from the shape of the antennae of butterflies.

The origins of the common names "butterfly" and "moth" are varied and often obscure. The English word butterfly is from Old English buttorfleoge, with many variations in spelling. Other than that, the origin is unknown, although it could be derived from the pale yellow color of many species' wings suggesting the color of butter. The species of Heterocera are commonly called moths. The origins of the English word moth are clearer, deriving from Old English moððe (cf. Northumbrian dialect mohðe) from Common Germanic (compare Old Norse motti, Dutch mot and German Motte all meaning "moth"). Perhaps its origins are related to Old English maða meaning "maggot" or from the root of "midge", which until the 16th century was used mostly to indicate the larva, usually in reference to devouring clothes.

The etymological origins of the word "caterpillar", the larval form of butterflies and moths, are from the early 16th century, from Middle English catirpel, catirpeller, probably an alteration of Old North French catepelose (from Latin cattus, "cat" + pilosus, "hairy").

The Lepidoptera are among the most successful groups of insects. They are found on all continents, except Antarctica, and inhabit all terrestrial habitats ranging from desert to rainforest, from lowland grasslands to mountain plateaus, but almost always associated with higher plants, especially angiosperms (flowering plants). Among the most northern dwelling species of butterflies and moths is the Arctic Apollo (Parnassius arcticus), which is found in the Arctic Circle in northeastern Yakutia, at an altitude of 1,500 metres (4,900 ft) above sea level. In the Himalayas, various Apollo species such as Parnassius epaphus have been recorded to occur up to an altitude of 6,000 metres (20,000 ft) above sea level.